test(scripts): add comprehensive unit tests for Update-ActionSHAPinning.ps1

- add API mocking infrastructure for GitHub API calls
- add Invoke-GitHubAPIWithRetry tests for retry logic and exponential backoff
- add Write-OutputResult tests for all 5 output formats
- add Get-LatestCommitSHA and Test-GitHubToken tests
- add Export-SecurityReport, Set-ContentPreservePermission, Add-SecurityIssue, Write-SecurityLog tests

🧪 - Generated by Copilot

Author: WilliamBerryiii