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/personality.rs

@@ -613,6 +613,7 @@ async fn ignores_remote_personality_if_remote_models_disabled() -> anyhow::Resul
         effective_context_window_percent: 95,
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
     };
 
     let _models_mock = mount_models_once(
@@ -729,6 +730,7 @@ async fn remote_model_friendly_personality_instructions_with_feature() -> anyhow
         effective_context_window_percent: 95,
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
     };
 
     let _models_mock = mount_models_once(
@@ -840,6 +842,7 @@ async fn user_turn_personality_remote_model_template_includes_update_message() -
         effective_context_window_percent: 95,
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
     };
 
     let _models_mock = mount_models_once(