fix: resolve code fixer merge conflict markers in multi-target projects

[thomhurst]

Summary

• Fix namespace verification in migration analyzers to handle partial semantic resolution failures
• Ensure consistent code fixer output across different TFMs in multi-targeting projects

Problem

When running migration code fixers (TUNU0001, TUXU0001) on multi-targeting projects, Roslyn inserts merge conflict markers when different TFMs produce different transformation outputs:

<<<<<<< TODO: Unmerged change from project 'Project(net462)', Before:
Assert.AreEqual(4, expected);
=======
Assert.AreEqual(expected).IsEqualTo(4);
>>>>>>> After
await Assert.That(expected).IsEqualTo(4);

This is a known Roslyn/dotnet format behavior. When semantic analysis fails for some TFMs (due to different package versions, missing dependencies, or inconsistent semantic models), the namespace verification methods return false instead of true, causing different transformation outputs per TFM.

Solution

Modified the namespace verification methods in all three migration analyzers to only return false when we positively know the namespace does NOT match the expected framework:

• XUnitTwoPhaseAnalyzer: IsXUnitAssertion() now returns true if namespace is null or matches "Xunit.Assert"
• NUnitTwoPhaseAnalyzer: VerifyNUnitNamespace() now returns true if namespace is null or matches "NUnit.Framework"
• MSTestTwoPhaseAnalyzer: VerifyMSTestNamespace() now returns true if namespace is null or matches "Microsoft.VisualStudio.TestTools.UnitTesting"

Test plan

• Build passes
• xUnit migration tests pass (177 passed, 3 skipped)
• NUnit migration tests pass (411 passed)
• MSTest migration tests pass (117 passed)

Fixes #4612
Fixes #4613

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.