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/core/tests/suite/model_switching.rs

@@ -225,6 +225,7 @@ async fn model_change_from_image_to_text_strips_prior_image_content() -> Result<
         visibility: ModelVisibility::List,
         supported_in_api: true,
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
         priority: 1,
         upgrade: None,
         base_instructions: "base instructions".to_string(),