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/app-server/tests/common/models_cache.rs

@@ -40,6 +40,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         effective_context_window_percent: 95,
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
+        prefer_websockets: false,
     }
 }