fix: harden GitLab merge request handling

[eshulman2]

Summary

• Restore the GitLab source-control integration from Add GitLab adapter #314 on the main-based branch.
• Normalize configured self-managed GitLab API URLs and document accepted forms.
• Preserve REST merge-request head SHAs and source-project cache keys.
• Treat allowed GitLab job failures as neutral and make 409 merge-request recovery fork-safe.

Validation

• 195 targeted GitLab and source-control tests passed.
• Ruff passed for all changed Python files.
• git diff --check passed.

Note

The full suite was attempted but timed out after 55 seconds at the previously isolated, environment-specific synchronous FastAPI TestClient hang in the GitLab webhook route tests; no assertion failure was reported.