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/rmcp_client.rs

@@ -409,6 +409,7 @@ async fn stdio_image_responses_are_sanitized_for_text_only_model() -> anyhow::Re
                 effective_context_window_percent: 95,
                 experimental_supported_tools: Vec::new(),
                 input_modalities: vec![InputModality::Text],
+                prefer_websockets: false,
             }],
         },
     )