Fix MSTEST0037 to detect LINQ Enumerable.Contains on interface types

[Evangelink]

MSTEST0037 (UseProperAssertMethodsAnalyzer) was not detecting Assert.IsTrue(collection.Contains(item)) when .Contains() resolved to the LINQ extension method System.Linq.Enumerable.Contains() instead of an instance method on the collection type. This affected interface types like IReadOnlyList, IReadOnlyCollection, and IEnumerable.

Root cause: RecognizeCollectionMethodCheck only handled instance methods (Parameters.Length == 1, ContainingType is the collection). For LINQ calls, Roslyn represents them with ContainingType == Enumerable and Arguments includes the 'this' parameter (Length == 2).

Fix: Add a second detection path that matches Enumerable.Contains calls by checking ContainingType against the Enumerable type symbol, matching the pattern already used by TryMatchLinqMethod for Any()/Count().

Fixes #7486

[Copilot]

Pull request overview

This PR fixes MSTEST0037 (UseProperAssertMethodsAnalyzer) to detect Assert.IsTrue(collection.Contains(item)) when .Contains() resolves to the LINQ extension method System.Linq.Enumerable.Contains<T>() instead of an instance method. This was previously missed for interface types like IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, and custom non-BCL types that don't define their own Contains.

Changes:

• Adds a new LINQ detection path in RecognizeCollectionMethodCheck that matches Enumerable.Contains calls by checking ContainingType == enumerableTypeSymbol and Arguments.Length == 2
• Propagates enumerableTypeSymbol into the RecognizeCollectionMethodCheck method signature
• Adds 6 new unit tests covering IReadOnlyList<T>, IEnumerable<T>, IReadOnlyCollection<T>, custom non-BCL collections, IsFalse variant, and a case with a message parameter

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Analyzers/MSTest.Analyzers/UseProperAssertMethodsAnalyzer.cs	Adds LINQ Enumerable.Contains detection path in RecognizeCollectionMethodCheck by checking for ContainingType == enumerableTypeSymbol with 2-argument form
test/UnitTests/MSTest.Analyzers.UnitTests/UseProperAssertMethodsAnalyzerTests.cs	Adds 6 unit tests verifying the new LINQ Contains detection and code fix for various interface types and scenarios

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[Copilot]

The test method name WhenAssertIsTrueWithLinqContainsOnCustomCollection_StillReports is misleading. The "StillReports" suffix implies pre-existing behavior, but this is actually a new scenario enabled by the fix. For non-BCL custom types, the old instance method path would NOT have reported a diagnostic (it requires IsBCLCollectionType), but now the new LINQ Enumerable.Contains path fires for ANY type. A more accurate name would be WhenAssertIsTrueWithLinqContainsOnCustomNonBCLCollection_Reports or similar.