Reduce Validation.cs duplication and improve code coverage

[Copilot]

Sonar flagged two quality issues: high duplicate-line density in Validation.cs and insufficient code coverage.

Duplicate Code Reduction

Extracted a RunValidationTest helper method following the SarifMark pattern, replacing ~130 lines of near-identical boilerplate across the three test methods with a single shared implementation that accepts a lambda validator:

private static void RunValidationTest(
    Context context,
    DemaConsulting.TestResults.TestResults testResults,
    string testName,
    string displayName,
    string[] additionalArgs,
    Func<string, string?> validator)

Each specific test now only provides its unique args and validation logic:

private static void RunHelpTest(Context context, DemaConsulting.TestResults.TestResults testResults)
{
    RunValidationTest(context, testResults, "NuGetCache_HelpDisplay", "Help Display Test",
        ["--help"],
        logContent => logContent.Contains("Usage:") && logContent.Contains("Options:")
            ? null : "Help text not found in log");
}

Coverage Improvements

Added 6 new tests covering previously untested paths:

• Context_WriteError_SetsErrorExitCode — WriteError → ExitCode == 1
• Context_WriteError_NotSilent_WritesToConsole — WriteError writes to stdout
• Context_Create_LogFlag_WithoutValue_ThrowsArgumentException — missing --log value
• Context_Create_ResultsFlag_WithoutValue_ThrowsArgumentException — missing --results value
• IntegrationTest_ValidateWithResults_GeneratesJUnitFile — .xml (JUnit) output branch of WriteResultsFile, placed in IntegrationTests.cs alongside the existing TRX test for consistency
• Program_Run_WithValidateAndUnsupportedResultsFormat_SetsErrorExitCode — unsupported extension error path

Test count: 35 → 41.

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
• Self-validation tests pass:
  dotnet run --project src/DemaConsulting.NuGet.CacheTool --configuration Release --framework net10.0
--no-build -- --validate
• 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

CodeQL scan returned 0 alerts.

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