## Summary
- deserialize `retry_model` from streamed `safety_buffering` payloads
- preserve the existing downstream faster-model API and legacy header
fallback
- update SSE, WebSocket, and end-to-end safety-buffering coverage
## Root cause
Follow-up to #31064. The Responses API emits the retry target as
`retry_model`, but the client was looking for `faster_model`, so the
payload value was ignored in favor of the compatibility fallback.
## Behavior
A non-null `retry_model` from the buffering payload takes precedence. An
explicit null leaves the retry target unset, while an omitted field
continues to fall back to the existing response header.
## Validation
- `just test -p codex-api` (135 tests)
- `just test -p codex-core safety_buffering` (2 tests)
- `just fix -p codex-api`
- `just fmt`
- `git diff --check`
Francis Chalissery
7094fa467e