The api_server adapter returned error code "invalid_api_key" for
API_SERVER_KEY authentication failures, which the Desktop error
classifier misidentified as a provider (OpenRouter/OpenAI) key
problem — showing "OpenRouter API key missing" when the real issue
was gateway auth.
Changes:
- gateway/platforms/api_server.py: return "gateway_auth_failed" code
with descriptive message for API_SERVER_KEY auth failures
- apps/desktop/src/store/notifications.ts: add "gateway_auth_failed"
handler before "invalid_api_key" to show correct error message
- agent/error_classifier.py: add "gateway_auth_failed" to auth patterns
- tests: update test_session_api.py to expect new error code
Fixes#39365