Change "cumulative input is required" error to be a warning (#412)

Towards #387

### Description

Relaxes the validation error for providing no input to a cumulative metric (no measure nor metric) to be a warning instead of an error to avoid breaking changes.

Context: previously, we weren't checking this, so making this an error could break existing manifests that work in general and are being used to run jobs other than this cumulative metric.  (The metric will not work in metricflow without an input.)

### Checklist

- [x] I have read [the contributing guide](https://github.com/dbt-labs/dbt-semantic-interfaces/blob/main/CONTRIBUTING.md) and understand what's expected of me
- [x] I have signed the [CLA](https://docs.getdbt.com/docs/contributor-license-agreements)
- [x] This PR includes tests, or tests are not required/relevant for this PR
- [x] I have run `changie new` to [create a changelog entry](https://github.com/dbt-labs/dbt-semantic-interfaces/blob/main/CONTRIBUTING.md#adding-a-changelog-entry)

Author: theyostalservice

.changes/unreleased/Under the Hood-20250909-102450.yaml

@@ -0,0 +1,6 @@
+kind: Under the Hood
+body: Relaxes a validation error for providing no input to a cumulative metric (neither measure nor metric) to be a warning instead of an error to avoid breaking.
+time: 2025-09-09T10:24:50.225208-07:00
+custom:
+    Author: theyostalservice
+    Issue: "387"

dbt_semantic_interfaces/validations/metrics.py

@@ -80,7 +80,7 @@ def _validate_input_measure_xor_metric(cls, metric: Metric) -> Sequence[Validati
             )
         elif metric.type_params.measure is None and input_metric is None:
             issues.append(
-                ValidationError(
+                ValidationWarning(
                     context=MetricContext(
                         file_context=FileContext.from_metadata(metadata=metric.metadata),
                         metric=MetricModelReference(metric_name=metric.name),

tests/validations/test_metrics.py

@@ -1197,7 +1197,7 @@ def test_cumulative_metrics() -> None:  # noqa: D
 
 
 @pytest.mark.parametrize(
-    "metric, error_substring_if_error",
+    "metric, error_substring_if_error, error_substring_if_warning",
     [
         (
             metric_with_guaranteed_meta(
@@ -1209,6 +1209,7 @@ def test_cumulative_metrics() -> None:  # noqa: D
                 ),
             ),
             None,  # No error; this should pass
+            None,
         ),
         (
             metric_with_guaranteed_meta(
@@ -1222,6 +1223,7 @@ def test_cumulative_metrics() -> None:  # noqa: D
                 ),
             ),
             None,  # No error; this should pass
+            None,
         ),
         (
             metric_with_guaranteed_meta(
@@ -1237,6 +1239,7 @@ def test_cumulative_metrics() -> None:  # noqa: D
             ),
             "Cumulative metric 'bad_metric_has_both_measure_and_metric_as_inputs' cannot have both a measure "
             "and a metric as inputs.  Please remove one of them.",
+            None,
         ),
         (
             metric_with_guaranteed_meta(
@@ -1246,6 +1249,7 @@ def test_cumulative_metrics() -> None:  # noqa: D
                     cumulative_type_params=PydanticCumulativeTypeParams(period_agg=PeriodAggregation.FIRST),
                 ),
             ),
+            None,
             "Cumulative metric 'bad_metric_has_neither_measure_nor_metric_as_inputs' must have either a measure "
             "or a metric as inputs. Please add one of them.",
         ),
@@ -1254,6 +1258,7 @@ def test_cumulative_metrics() -> None:  # noqa: D
 def test_cumulative_metrics_have_metric_xor_measure(
     metric: PydanticMetric,
     error_substring_if_error: Optional[str],
+    error_substring_if_warning: Optional[str],
 ) -> None:
     """Validate that things like fill_nulls_with and join_to_timespine are not allowed on non-simple metrics."""
     model_validator = SemanticManifestValidator[PydanticSemanticManifest]([CumulativeMetricRule()])
@@ -1294,8 +1299,10 @@ def test_cumulative_metrics_have_metric_xor_measure(
         )
     )
     if error_substring_if_error:
-        check_error_in_issues(error_substrings=[error_substring_if_error], issues=validation_results.all_issues)
-    else:
+        check_error_in_issues(error_substrings=[error_substring_if_error], issues=validation_results.errors)
+    if error_substring_if_warning:
+        check_error_in_issues(error_substrings=[error_substring_if_warning], issues=validation_results.warnings)
+    if not error_substring_if_error and not error_substring_if_warning:
         assert len(validation_results.all_issues) == 0, "expected this metric to pass validation, but it did not"