Resolve conflicts with main:
- service/grok_media.go: keep both post-response blocks — #4497's empty
image-output failover (main) runs first for image endpoints, then this
branch's video-status content-URL rewrite; the endpoint conditions are
mutually exclusive
- handler/openai_gateway_credential_failover_loop_test.go: mark the stub
OAuth accounts media-eligible via the grok_media_eligible extra override,
because this branch moved grok media failover coverage to the generation
endpoint, which is now gated by #4540's paid-eligibility probe on main
shaw