Prefer websocket transport when model opts in (#11386)

Summary
- add a `prefer_websockets` field to `ModelInfo`, defaulting to `false`
in all fixtures and constructors
- wire the new flag into websocket selection so models that opt in
always use websocket transport even when the feature gate is off

Testing
- Not run (not requested)

codex-rs/codex-api/tests/models_integration.rs

@@ -88,6 +88,7 @@ async fn models_client_hits_models_endpoint() {
             effective_context_window_percent: 95,
             experimental_supported_tools: Vec::new(),
             input_modalities: default_input_modalities(),
+            prefer_websockets: false,
         }],
     };