Integrate twin-openai (fake OpenAI server) into the workspace and wire
it into the e2e_test macro so OpenAI tests can run without real API
credentials. The twin server starts in-process via OnceLock on first use
and provides per-test isolation through bearer-token namespacing.
Changes:
- Add Twin as default TestMode, replacing Off (gating now via #[ignore])
- Extend #[e2e_test] macro with `twin` requirement for twin-only,
live-only, and dual-mode (twin + live) test gating
- Add e2e_openai!() macro returning (base_url, api_key)
- Convert openai_complete and openai_gpt_5_3_codex_complete to dual-mode
- Add new openai_server_error twin-only test with scripted 500 error
- Standardize axum 0.8 as workspace dependency across all crates
- Relax twin-openai ResponsesRequest to accept unknown fields via flatten
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
