Fix stale incremental generator cache when referenced types change in other source files

[Copilot]

MemberDeclarationSyntaxAndCompilationEqualityComparer only checked external assembly references, so changes to types in other source files (e.g., an enum gaining/losing members) were invisible to the cache — the generator reused stale output even though the semantic model had changed.

Changes

Bug fix — MemberDeclarationSyntaxAndCompilationEqualityComparer

• Added source-tree reference equality check (step 5) before the external-references check.
  Roslyn reuses the same SyntaxTree object for unchanged files, so comparing by reference is cheap (O(n) pointer comparisons) and semantically correct.
• Added a file-scoped ReferenceEqualityTreeComparer helper (netstandard2.0-compatible).

// Before — only external assemblies were checked:
return xCompilation.ExternalReferences.SequenceEqual(yCompilation.ExternalReferences);

// After — source trees checked first, then external references:
if (!xCompilation.SyntaxTrees.SequenceEqual(yCompilation.SyntaxTrees, ReferenceEqualityTreeComparer.Instance))
    return false;
return xCompilation.ExternalReferences.SequenceEqual(yCompilation.ExternalReferences);

New test helpers — ProjectionExpressionGeneratorTestsBase

• GetDefaultReferences() — extracted so incremental tests can build multi-file compilations without duplicating the #if NET10_0/NET8_0 reference boilerplate.
• CreateAndRunGenerator() — creates a fresh driver, returns (driver, result) to seed incremental state.
• RunGeneratorWithDriver(driver, compilation) — reruns an existing driver on an updated compilation, preserving cache state.

New tests — IncrementalGeneratorTests

Tests that reuse a GeneratorDriver across two compilations to guard against cache regressions:

Test	What it verifies
WhenReferencedEnumInAnotherFileGainsAMember_GeneratedOutputUpdates	New enum member propagates into the ternary expansion
WhenReferencedEnumInAnotherFileLosesAMember_GeneratedOutputUpdates	Removed enum member is no longer present in output
WhenProjectableFileChanges_GeneratedOutputUpdates	Direct changes to the projectable still re-run (sanity)
WhenCompilationIsUnchanged_GeneratorProducesSameOutput	Cache is still valid when nothing changed

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