core tests: use hermetic mock server in review suite (#12291)

## Summary
- switch the review test SSE mock helper to use the shared hermetic mock
server setup
- ensure review tests always have a default `/v1/models` stub during
Codex session bootstrap
- remove the race that caused intermittent `/v1/models` connection
failures and flaky ETag refresh assertions

## Testing
- `just fmt`
- `cargo test -p codex-core --test all
refresh_models_on_models_etag_mismatch_and_avoid_duplicate_models_fetch`
- `cargo test -p codex-core --test all
review_uses_custom_review_model_from_config`
- repeated both targeted tests 5x in a loop
- `cargo clippy -p codex-core --tests -- -D warnings`