Add snippet validation to integrity checks workflow

[Copilot]

Documentation snippets are sourced from C# tests. This adds automated validation that Update-Snippets.ps1 has been run when snippet tests or docs change.

Changes

• Renamed workflow: codegen-validation.yml → integrity-checks.yml to reflect expanded scope
• Created composite action: .github/actions/check-uncommitted-changes/action.yml
  • Parameterized error message
  • Checks both tracked changes (git diff) and untracked files
• Refactored existing jobs: validate-codegen and validate-api-export now use composite action (eliminates 30+ lines of duplication)
• Added validate-snippets job:
  • Runs Update-Snippets.ps1
  • Fails if uncommitted changes detected
  • Triggers on changes to tests/Snippets/**, README.md, or the update script

Workflow trigger paths updated

paths:
  - '.github/workflows/integrity-checks.yml'  # was codegen-validation.yml
  - 'tests/Snippets/**'                       # new
  - 'README.md'                               # new
  - 'scripts/Update-Snippets.ps1'            # new

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