fix(api): do not rely on inexact volume in tip to determine last dispense (#19030)

# Overview

In `distribute_with_liquid_class()`, after we dispense to the last well
of a multi-dispense, we're supposed to blow out the disposal volume.
This is triggered by calling
`retract_during_multi_dispensing(is_last_retract=True)`.

However, we were computing `is_last_retract` as:
```
components_executor.retract_during_multi_dispensing(
    is_last_retract=tip_starting_volume - volume == disposal_volume,
)
```
where `tip_starting_volume` is the current volume in the tip before the
dispense, `volume` is the amount we're about to dispense, and
`disposal_volume` is the disposal volume.

This equality becomes **inexact** when `tip_starting_volume`, `volume`,
and `disposal_volume` are not nice round numbers, which often happens
with liquid classes because the `disposal_volume` is interpolated.

This floating point bug causes `is_last_retract` to be `False`, which
causes us to never dispose of the disposal volume. (AUTH-2151)

This fix changes the code to no longer rely on `tip_starting_volume` to
determine whether we are about to do the last dispense. Instead, in the
loop that performs the multi-dispense, we simply see if we're in the
last iteration of the loop.

## Test Plan and Hands on Testing

I updated the unit tests. Note that we already have existing unit tests
for both the `is_last_retract=False` and `is_last_retract=True` cases.

After my change, I was able to delete
`decoy.when(get_aspirated_volume(...)).then_return(...)` from the unit
tests, which ensures that our implementation no longer looks at the
current volume in the tip.

I also manually analyzed this protocol to check that we are blowing out
the disposal volume:
```
from opentrons import protocol_api, types

requirements = {"robotType": "Flex", "apiLevel": "2.24"}

def run(protocol: protocol_api.ProtocolContext) -> None:
    tip_rack = protocol.load_labware(
        "opentrons_flex_96_tiprack_200ul",
        location="C2",
        namespace="opentrons",
    )
    well_plate = protocol.load_labware(
        "opentrons_96_wellplate_200ul_pcr_full_skirt",
        location="B1",
        namespace="opentrons",
    )
    pipette = protocol.load_instrument(
        "flex_1channel_1000", "left", tip_racks=[tip_rack],
    )
    waste_chute = protocol.load_waste_chute()

    liquid_class = protocol.get_liquid_class(name="water")
    transfer_props = liquid_class.get_for(pipette, tip_rack)
    transfer_props.multi_dispense.retract.blowout.location = "trash"
    # Before this fix, we would not blow out this disposal volume:
    transfer_props.multi_dispense.disposal_by_volume.set_for_all_volumes(1.1)

    pipette.distribute_with_liquid_class(
        liquid_class=liquid_class,
        volume=10,
        source=[well_plate["A1"]],
        dest=[well_plate["G11"], well_plate["H12"]],
        new_tip="once",
        return_tip=True,
    )
```

## Risk assessment

Medium. This is rather complicated code.

Author: ddcc4

api/src/opentrons/protocol_api/core/engine/instrument.py

@@ -1705,7 +1705,7 @@ def _pick_up_tip() -> Tuple[Location, WellCore]:
             # multi-dispense in those destinations.
             # If the tip has a volume corresponding to a single destination, then
             # do a single-dispense into that destination.
-            for dispense_vol, dispense_dest in vol_dest_combo:
+            for idx, (dispense_vol, dispense_dest) in enumerate(vol_dest_combo):
                 if use_single_dispense:
                     tip_contents = self.dispense_liquid_class(
                         volume=dispense_vol,
@@ -1733,6 +1733,7 @@ def _pick_up_tip() -> Tuple[Location, WellCore]:
                         trash_location=trash_location,
                         conditioning_volume=conditioning_vol,
                         disposal_volume=disposal_vol,
+                        is_last_dispense_in_tip=(idx == len(vol_dest_combo) - 1),
                     )
                 is_first_step = False
 
@@ -2286,6 +2287,7 @@ def dispense_liquid_class_during_multi_dispense(
         trash_location: Union[Location, TrashBin, WasteChute],
         conditioning_volume: float,
         disposal_volume: float,
+        is_last_dispense_in_tip: bool,
     ) -> List[tx_comps_executor.LiquidAndAirGapPair]:
         """Execute a dispense step that's part of a multi-dispense.
 
@@ -2332,9 +2334,8 @@ def dispense_liquid_class_during_multi_dispense(
         components_executor.submerge(
             submerge_properties=dispense_props.submerge, post_submerge_action="dispense"
         )
-        tip_starting_volume = self.get_current_volume()
         is_last_dispense_without_disposal_vol = (
-            disposal_volume == 0 and tip_starting_volume == volume
+            disposal_volume == 0 and is_last_dispense_in_tip
         )
         push_out_vol = (
             # TODO (spp): verify if it's okay to use push_out_by_volume of single dispense
@@ -2354,7 +2355,7 @@ def dispense_liquid_class_during_multi_dispense(
             source_well=source[1] if source else None,
             conditioning_volume=conditioning_volume,
             add_final_air_gap=add_final_air_gap,
-            is_last_retract=tip_starting_volume - volume == disposal_volume,
+            is_last_retract=is_last_dispense_in_tip,
         )
         last_contents = components_executor.tip_state.last_liquid_and_air_gap_in_tip
         new_tip_contents = tip_contents[0:-1] + [last_contents]

api/tests/opentrons/protocol_api/core/engine/test_instrument_core.py

@@ -2666,9 +2666,6 @@ def test_dispense_liquid_class_during_multi_dispense(
     decoy.when(
         mock_transfer_components_executor.tip_state.last_liquid_and_air_gap_in_tip
     ).then_return(LiquidAndAirGapPair(liquid=333, air_gap=444))
-    decoy.when(
-        mock_engine_client.state.pipettes.get_aspirated_volume("abc123")
-    ).then_return(12345)
     result = subject.dispense_liquid_class_during_multi_dispense(
         volume=123,
         dest=(dest_location, dest_well),
@@ -2680,6 +2677,7 @@ def test_dispense_liquid_class_during_multi_dispense(
         trash_location=Location(Point(1, 2, 3), labware=None),
         conditioning_volume=conditioning_volume,
         disposal_volume=disposal_volume,
+        is_last_dispense_in_tip=False,  # testing the case when this is not last dispense
     )
     decoy.verify(
         mock_transfer_components_executor.submerge(
@@ -2750,9 +2748,6 @@ def test_last_dispense_liquid_class_during_multi_dispense(
     decoy.when(
         mock_transfer_components_executor.tip_state.last_liquid_and_air_gap_in_tip
     ).then_return(LiquidAndAirGapPair(liquid=333, air_gap=444))
-    decoy.when(
-        mock_engine_client.state.pipettes.get_aspirated_volume("abc123")
-    ).then_return(123)
     result = subject.dispense_liquid_class_during_multi_dispense(
         volume=123,
         dest=(dest_location, dest_well),
@@ -2764,6 +2759,7 @@ def test_last_dispense_liquid_class_during_multi_dispense(
         trash_location=Location(Point(1, 2, 3), labware=None),
         conditioning_volume=conditioning_volume,
         disposal_volume=disposal_volume,
+        is_last_dispense_in_tip=True,
     )
     decoy.verify(
         mock_transfer_components_executor.submerge(

api/tests/opentrons/protocol_api_integration/test_transfer_with_liquid_classes.py

@@ -1512,6 +1512,7 @@ def test_order_of_water_distribution_steps_using_multi_dispense(
                 trash_location=mock.ANY,
                 conditioning_volume=expected_conditioning_volume,
                 disposal_volume=expected_disposal_volume,
+                is_last_dispense_in_tip=False,
             ),
             mock.call.dispense_liquid_class_during_multi_dispense(
                 mock.ANY,
@@ -1532,6 +1533,7 @@ def test_order_of_water_distribution_steps_using_multi_dispense(
                 trash_location=mock.ANY,
                 conditioning_volume=expected_conditioning_volume,
                 disposal_volume=expected_disposal_volume,
+                is_last_dispense_in_tip=True,
             ),
             mock.call.aspirate_liquid_class(
                 mock.ANY,
@@ -1562,6 +1564,7 @@ def test_order_of_water_distribution_steps_using_multi_dispense(
                 trash_location=mock.ANY,
                 conditioning_volume=expected_conditioning_volume,
                 disposal_volume=expected_disposal_volume,
+                is_last_dispense_in_tip=False,
             ),
             mock.call.dispense_liquid_class_during_multi_dispense(
                 mock.ANY,
@@ -1582,6 +1585,7 @@ def test_order_of_water_distribution_steps_using_multi_dispense(
                 trash_location=mock.ANY,
                 conditioning_volume=expected_conditioning_volume,
                 disposal_volume=expected_disposal_volume,
+                is_last_dispense_in_tip=True,
             ),
             mock.call.drop_tip_in_disposal_location(
                 mock.ANY,
@@ -2108,6 +2112,7 @@ def test_order_of_water_distribution_steps_using_mixed_dispense(
                 trash_location=mock.ANY,
                 conditioning_volume=expected_conditioning_volume,
                 disposal_volume=expected_disposal_volume,
+                is_last_dispense_in_tip=False,
             ),
             mock.call.dispense_liquid_class_during_multi_dispense(
                 mock.ANY,
@@ -2125,6 +2130,7 @@ def test_order_of_water_distribution_steps_using_mixed_dispense(
                 trash_location=mock.ANY,
                 conditioning_volume=expected_conditioning_volume,
                 disposal_volume=expected_disposal_volume,
+                is_last_dispense_in_tip=True,
             ),
             mock.call.aspirate_liquid_class(
                 mock.ANY,