refactor: Consolidate test source code to eliminate DRY violations

[thomhurst]

Summary

• Creates TestSourceConstants.cs with shared using statements and namespace constants for analyzer tests
• Creates SerializationTestModels.cs with shared test models (TestValues, SerializationTestModel)
• Updates analyzer tests to use shared constants instead of duplicated strings
• Updates serialization tests to use shared models

Fixes #1634, #1627

Test plan

• Build passes
• All tests pass

🤖 Generated with Claude Code

[thomhurst]

Summary

Refactors analyzer and serialization unit tests to eliminate code duplication by extracting shared constants and models.

Critical Issues

None found ✅

Suggestions

None - the refactoring is well-executed with proper curly brace escaping in interpolated strings and maintains all test semantics.

Verdict

✅ APPROVE - No critical issues

This is a clean refactoring that:

• Reduces 751 lines while adding only 423, net -328 lines
• Properly escapes curly braces for C# interpolated verbatim strings ($@"...")
• Correctly handles Roslyn analyzer test markup syntax ({{|#0:...|}})
• Maintains semantic equivalence across all test cases
• Improves maintainability by centralizing common test patterns

The shared constants approach will make future test maintenance significantly easier.

[thomhurst]

Summary

Consolidates duplicated test code by extracting shared constants and test models into dedicated files.

Critical Issues

None found ✅

Suggestions

String interpolation escaping: In several test files, the refactoring uses string interpolation with constants that contain interpolation markers like {|#0:...|}. This pattern works correctly, but be aware that:

• The constants use $@"..." for verbatim interpolated strings
• The diagnostic markers {{|#0:...|}} use double braces to escape them
• This is intentional and correct, but verify the tests still pass to ensure the escaping is consistent

Test coverage: The refactoring touches 7 analyzer test files and 3 serialization test files. Ensure all tests pass after this change, as incorrect escaping could cause test failures.

Verdict

✅ APPROVE - No critical issues

This is a clean refactoring that eliminates significant code duplication. The use of shared constants makes the tests more maintainable and easier to update in the future. The string interpolation approach is sound, just verify all tests pass to confirm the escaping is correct.