The 403 detection PR changed the 401 handler condition from
`account.Type == AccountTypeOAuth` to
`account.Type == AccountTypeOAuth && account.Platform == PlatformOpenAI`,
which accidentally excluded Gemini OAuth from the temp-unschedulable path.
Fix: use `!= PlatformAntigravity` instead, preserving Gemini behavior
while correctly excluding Antigravity (whose 401 is handled by
applyErrorPolicy's temp_unschedulable_rules).
Update tests to reflect Antigravity's new 401 semantics:
- HandleUpstreamError: Antigravity OAuth 401 now uses SetError
- CheckErrorPolicy: Antigravity 401 second hit stays TempUnscheduled
- DB fallback: split into Gemini (escalates) and Antigravity (stays temp)
When custom error codes are enabled and the upstream error code is NOT
in the configured list, return HTTP 500 to the client instead of
transparently forwarding the original status code.
Also adds integration test TestCustomErrorCode599 verifying that 429,
500, 503, 401, 403 all return 500 without triggering SetRateLimited
or SetError.
