fix: correct xUnit Assert.Same/NotSame conversion method names

[thomhurst]

Summary

• Fixed xUnit migration code fixer generating incorrect method names for reference equality assertions
• Changed IsSameReference → IsSameReferenceAs and IsNotSameReference → IsNotSameReferenceAs
• Added tests for Assert.Same and Assert.NotSame conversion

Test plan

• Added Assert_Same_Can_Be_Converted test
• Added Assert_NotSame_Can_Be_Converted test
• All existing xUnit migration tests pass (41/41)

Fixes #4333

🤖 Generated with Claude Code

[thomhurst]

Summary

Fixes xUnit migration code fixer to generate correct method names for reference equality assertions (IsSameReferenceAs and IsNotSameReferenceAs instead of non-existent IsSameReference/IsNotSameReference).

Critical Issues

None found ✅

Review Notes

Correctness Verified:

• ✅ Method names now match actual TUnit API in TUnit.Assertions/Conditions/ReferenceEqualityAssertions.cs:10,47
• ✅ Consistent with NUnit migration code fixer in TUnit.Analyzers.CodeFixers/NUnitMigrationCodeFixProvider.cs:919-920
• ✅ Argument order preserved correctly (actual, expected) → Assert.That(actual).IsSameReferenceAs(expected)
• ✅ Both Assert.Same and Assert.NotSame conversions tested with comprehensive test cases

TUnit Rules Compliance:

• ✅ No source generator changes → snapshot tests not required
• ✅ No public API changes → API snapshot tests not required
• ✅ Changes only affect analyzers/code fixers → no dual-mode or reflection concerns
• ✅ No VSTest usage, no performance hotpaths affected

Testing:
The new tests correctly verify:

1. xUnit Assert.Same(expected, actual) → TUnit Assert.That(actual).IsSameReferenceAs(expected)
2. xUnit Assert.NotSame(obj1, obj2) → TUnit Assert.That(obj2).IsNotSameReferenceAs(obj1)

Both tests properly validate the complete migration including [Fact] → [Test] and void → async Task conversions.

Verdict

✅ APPROVE - Clean fix that resolves issue #4333 with proper test coverage and no rule violations.