Fix MSTEST0049 false positives inside expression trees (#7585)

[Evangelink]

Skip reporting MSTEST0049 diagnostics when the invocation is inside an expression tree (e.g. Moq Setup lambdas), where the code fix cannot be meaningfully applied.

• Add IsInsideExpressionTree helper that walks up the operation tree to detect lambdas converted to Expression
• Add SystemLinqExpressionsLambdaExpression to WellKnownTypeNames
• Add 3 tests covering expression tree suppression and regular lambda non-suppression

Fixes #7585

[Copilot]

Pull request overview

Updates the MSTEST0049 analyzer to avoid reporting diagnostics for invocations that occur inside expression trees (e.g., Moq Setup lambdas), where applying the code fix would be inappropriate.

Changes:

• Add expression-tree detection (IsInsideExpressionTree) to suppress MSTEST0049 diagnostics when the invocation is within an Expression<T> lambda.
• Extend WellKnownTypeNames with System.Linq.Expressions.LambdaExpression for symbol lookup.
• Add unit tests covering suppression inside expression trees and ensuring normal lambdas still produce diagnostics/fixes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/UnitTests/MSTest.Analyzers.UnitTests/FlowTestContextCancellationTokenAnalyzerTests.cs	Adds new tests validating expression-tree suppression and non-expression lambda behavior.
src/Analyzers/MSTest.Analyzers/Helpers/WellKnownTypeNames.cs	Adds a well-known type name constant for LambdaExpression.
src/Analyzers/MSTest.Analyzers/FlowTestContextCancellationTokenAnalyzer.cs	Introduces expression-tree detection and skips MSTEST0049 reporting within expression trees.