Add integration tests that directly exercise the PAPERCLIP_API_URL
conditional guard in startServer():
- Externally set PAPERCLIP_API_URL is preserved (not overwritten)
- Unset PAPERCLIP_API_URL falls back to host-based URL
Addresses Greptile review comment on PR #3472.
Co-Authored-By: Paperclip <noreply@paperclip.ing>