fix(logs): count a finished stream as success after disconnect

[caigee-cmd]

Summary

• A stream that already delivered its terminal frame is logged as success when the client disconnects afterward.
• A disconnect before the stream finishes stays canceled, and a real upstream failure stays an error.
• OpenAI, Anthropic, and Responses relays share the same status rule. A successful disconnect no longer stores context canceled on the request log.

Test plan

• go test ./internal/gateway/ ./internal/api/ -count=1
• After release, confirm new completed-then-closed streams show as ok instead of canceled on sgp3