Add shallow git checkout step in activation job for timestamp check

- Added sparse checkout of .github/workflows in activation job
- Uses fetch-depth: 1 for shallow clone to minimize performance impact
- Uses sparse-checkout-cone-mode: false for non-cone pattern support
- Sets persist-credentials: false for security
- Updated tests to check for checkout in agent job specifically
- Added new test TestActivationJobCheckoutStep to verify activation checkout
- Updated TestActivationAndAddReactionJobsPermissions to expect checkout in activation job
- Updated TestTrialModeCompilation to look for checkout in agent job

Co-authored-by: pelikhan <[email protected]>

Author: Copilot