Add Assert.StartsWith MSTest antipattern to test-developer agent guidance

[Copilot]

Pull Request

Description

Extends the test-developer agent's MSTest antipatterns list with guidance to prefer Assert.StartsWith over Assert.IsTrue(value.StartsWith(...)). The dedicated assertion produces richer failure output showing the expected prefix and actual value.

// ❌ Bad: Assert.IsTrue(value.StartsWith("prefix"));
// ✅ Good: Assert.StartsWith("prefix", value);

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code quality improvement

Related Issues

Pre-Submission Checklist

Before submitting this pull request, ensure you have completed the following:

Build and Test

• Code builds successfully: dotnet build --configuration Release
• All unit tests pass: dotnet test --configuration Release
• Code produces zero warnings

Code Quality

• Code formatting is correct: dotnet format --verify-no-changes
• New code has appropriate XML documentation comments
• Static analyzer warnings have been addressed

Quality Checks

Please run the following checks before submitting:

• Spell checker passes: cspell "**/*.{md,cs}"
• Markdown linter passes: markdownlint "**/*.md"
• YAML linter passes: yamllint .

Testing

• Added unit tests for new functionality
• Updated existing tests if behavior changed
• All tests follow the AAA (Arrange, Act, Assert) pattern
• Test coverage is maintained or improved

Documentation

• Updated README.md (if applicable)
• Updated ARCHITECTURE.md (if applicable)
• Added code examples for new features (if applicable)
• Updated requirements.yaml (if applicable)

Additional Notes

Documentation-only change to .github/agents/test-developer.md. No production code or tests affected.

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