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/remote_models.rs

@@ -141,6 +141,7 @@ async fn remote_models_remote_model_uses_unified_exec() -> 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(),
@@ -379,6 +380,7 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
         visibility: ModelVisibility::List,
         supported_in_api: true,
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
         priority: 1,
         upgrade: None,
         base_instructions: remote_base.to_string(),
@@ -862,6 +864,7 @@ fn test_remote_model_with_policy(
         visibility,
         supported_in_api: true,
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
         priority,
         upgrade: None,
         base_instructions: "base instructions".to_string(),