Plugin fixes and assists do not work inside `part of` files

[vowxzero]

Analyzer plugins can report diagnostics in part of files, but their quick-fixes and assists never appear there.
The same plugin fixes/assists work normally in the main library file.

This makes plugin behavior inconsistent with analyzer built-in diagnostics, which work in both library and part files.

Reproduction

test_part.dart

part "test_part_of.dart";

void testAwait() async {
  await Future.delayed(Duration(seconds: 1)); // works
  await Future.delayed(Duration(seconds: 1)); // works
  await Future.delayed(Duration(seconds: 1)); // works

  final s1 = '123'; // works
  final s2 = '456'; // works
  final se = '789'; // works
}

test_part_of.dart

part of "test_part.dart";

void testAwait2() async {
  await Future.delayed(Duration(seconds: 1)); // not working
  await Future.delayed(Duration(seconds: 1)); // not working
  await Future.delayed(Duration(seconds: 1)); // not working

  final s1 = '123'; // works
  final s2 = '456'; // works
  final se = '789'; // works
}

Behavior

File	Plugin Diagnostic	Plugin Fix	Plugin Assist
test_part.dart	✓ shown	✓ works	✓ works
test_part_of.dart	✓ shown	✗ missing	✗ missing

Diagnostics appear in both files, but only the main file receives fixes and assists.
The part file never offers any plugin-provided fix or assist.

Dart-SDK

• Dart 3.10.1 (stable) (Tue Nov 18 11:02:31 2025 -0800) on "macos_arm64"
• on macos / Version 26.0.1 (Build 25A362)

[FMorschel]

CC @srawlins @bwilkerson

I'll do some local testing.

[FMorschel]

I'm guessing these line should use getResolvedLibraryContaining instead, @srawlins. Would you like a CL here?

sdk/pkg/analysis_server_plugin/lib/src/plugin_server.dart

Line 133 in 0d052e3

var libraryResult = await analysisContext.currentSession.getResolvedLibrary(

sdk/pkg/analysis_server_plugin/lib/src/plugin_server.dart

Line 191 in 0d052e3

var libraryResult = await analysisContext.currentSession.getResolvedLibrary(

[srawlins]

A CL would be appreciated! Thank you

[FMorschel]

There is one more thing that is wrong. We save _recentState with the library path, but we need some reasonable work to get the session here, so we can find what the library path is. Should I change this _recentState to save the information by file, @srawlins (way easier)? Or getting the session and finding the library path is the preferred way?

[srawlins]

I think the easier way is just fine for this; thanks much!

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/464300