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/routes.py

@@ -7866,7 +7866,6 @@ from .upstream import (
     UpstreamTimeoutError,
     UpstreamHTTPError,
     UpstreamSSLError,
-    AirGapError,
     FileSizeExceededError as UpstreamFileSizeExceededError,
     SourceNotFoundError,
     SourceDisabledError,
@@ -8114,8 +8113,6 @@ def cache_artifact(
             cache_request.url,
             expected_hash=cache_request.expected_hash,
         )
-    except AirGapError as e:
-        raise HTTPException(status_code=403, detail=str(e))
     except SourceDisabledError as e:
         raise HTTPException(status_code=503, detail=str(e))
     except UpstreamHTTPError as e:

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.