Red main: AotConverterScanTests.SkipsConversionsBlockedByObsoleteError fails on net472 (IsExternalInit) #7329

Red main: SkipsConversionsBlockedByObsoleteError fails on net472

main run 41311511530 (commit d43977c, #7317), job modularpipeline (windows-latest), module RunSourceGeneratorTestsModule, TUnit.Core.SourceGenerator.Tests on net472:

failed SkipsConversionsBlockedByObsoleteError
  AssertionException: Expected to be empty
  but collection contains items: [(4,44): error CS0518: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported]

tests/TUnit.Core.SourceGenerator.Tests/AotConverterScanTests.cs compiles a fixture that declares public readonly record struct Token(string Value). A positional record's init accessor needs IsExternalInit, and the net472 reference set (ReferencesHelper.References) does not include it. The test passes on net8.0+ and fails on net472. Every PR inherits the failure (for example #7321).

Expected: the fixture compiles against every target framework of the test project.