refactor: remove duplicate git diff logic in frontmatter validator

[cnaples79]

Summary

• Removes duplicate Get-ChangedMarkdownFileGroup function from frontmatter validator
• Replaces with shared Get-ChangedFilesFromGit helper from LintingHelpers.psm1
• Removes obsolete test code for deleted function
• Updates remaining tests to mock the new shared function

Rationale

Issue #322 identified duplicated git diff logic. This refactoring eliminates 491 lines of duplicate code by reusing the shared linting helper.

Changes

• Source: Removed 104-line duplicate function, replaced with shared helper call
• Tests: Removed 386 lines of obsolete test code for deleted function
• Tests: Updated integration test mocks to use Get-ChangedFilesFromGit

Files changed:

• scripts/linting/Validate-MarkdownFrontmatter.ps1 (-106 lines)
• scripts/tests/linting/Validate-MarkdownFrontmatter.Tests.ps1 (-386 lines)

Validation

• All CI checks passing
• PowerShell tests pass
• Shared function already has test coverage in LintingHelpers.Tests.ps1

Fixes #322

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.50%. Comparing base (8133b36) to head (db265a2).

Files with missing lines	Patch %	Lines
scripts/linting/Validate-MarkdownFrontmatter.ps1	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #473      +/-   ##
==========================================
+ Coverage   76.22%   76.50%   +0.28%     
==========================================
  Files          20       20              
  Lines        3503     3478      -25     
==========================================
- Hits         2670     2661       -9     
+ Misses        833      817      -16     

Flag	Coverage Δ
pester	76.50% <0.00%> (+0.28%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
scripts/linting/Validate-MarkdownFrontmatter.ps1	70.05% <0.00%> (+4.28%)	⬆️

... and 1 file with indirect coverage changes

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[katriendg]

Thank you @cnaples79 for taking on this issue and contributing improvements.

With the current approach and deleted tests however, there are a few elements no longer tested in as much detail because LintingHelpers.Tests.ps1 does not cover some of what the deleted paths tested.

Could you give it another pass using either Task-Researcher or RPI to help you question which parts of the branch have removed tests that are now no longer/not yet covered, and should now be extended in the LintingHelpers.Tests.ps1.
This will ensure we don't lose any of the granularity of the testing which was previously there.