Adds 9 unit tests covering:
- yaml deserialization of flow: device_code
- merge preserves base device_authorization_url when user omits
- merge lets user device_authorization_url win
- merge lets user use_pkce_in_device_flow win
- OpenAICompatibleOAuthProvider exposes / defaults both new trait methods
- Full serde roundtrip of a realistic device_code yaml block
No network or polling — pure config/serde logic tests. Brings the test
count from 1134 to 1143.
Dark-Alex-17
5a5da90734