Fixes #634

Problem

When fetch_and_notify_on_changes fails (e.g., network timeout), the repository path (directory or symbolic link) is left in a corrupted state, causing subsequent fetches to fail indefinitely.

Solution

• Robust Cleanup: Wrapped the fetch logic in a try...except block that explicitly handles both os.unlink (for symlinks) and shutil.rmtree (for directories).
• Cache Clearing: Ensures the internal GitPolicyFetcher.repos cache is cleared on failure.
• Cross-Platform Test: Added a regression test (packages/opal-server/opal_server/tests/test_git_fetcher_cleanup.py) that mocks the failure and verifies filesystem cleanup. (Includes a Windows compatibility check for local dev).

Verification

I have added a new test file. Run it with: pytest packages/opal-server/opal_server/tests/test_git_fetcher_cleanup.py

/claim #634