Fix AirGapError import and remove remaining is_public references

- Remove AirGapError import from routes.py (class was deleted)
- Remove AirGapError exception handler from cache endpoint
- Remove AirGapError from docstring in upstream.py
- Remove TestUpstreamClientAirGapMode test class
- Remove is_public references from test fixtures and assertions

backend/app/upstream.py

@@ -278,7 +278,6 @@ class UpstreamClient:
             FetchResult with content, hash, size, and headers.
 
         Raises:
-            AirGapError: If air-gap mode blocks the request.
             SourceDisabledError: If the matching source is disabled.
             UpstreamConnectionError: On connection failures.
             UpstreamTimeoutError: On timeout.