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/client_websockets.rs

@@ -105,7 +105,7 @@ async fn responses_websocket_preconnect_reuses_connection() {
     let harness = websocket_harness(&server).await;
     let mut client_session = harness.client.new_session();
     client_session
-        .prewarm_websocket(&harness.otel_manager, None)
+        .prewarm_websocket(&harness.otel_manager, &harness.model_info, None)
         .await
         .expect("websocket prewarm failed");
     let prompt = prompt_with_input(vec![message_item("hello")]);
@@ -130,7 +130,7 @@ async fn responses_websocket_preconnect_is_reused_even_with_header_changes() {
     let harness = websocket_harness(&server).await;
     let mut client_session = harness.client.new_session();
     client_session
-        .prewarm_websocket(&harness.otel_manager, None)
+        .prewarm_websocket(&harness.otel_manager, &harness.model_info, None)
         .await
         .expect("websocket prewarm failed");
     let prompt = prompt_with_input(vec![message_item("hello")]);
@@ -158,6 +158,36 @@ async fn responses_websocket_preconnect_is_reused_even_with_header_changes() {
     server.shutdown().await;
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn responses_websocket_prewarm_uses_model_preference_when_feature_disabled() {
+    skip_if_no_network!();
+
+    let server = start_websocket_server(vec![vec![vec![
+        ev_response_created("resp-1"),
+        ev_completed("resp-1"),
+    ]]])
+    .await;
+
+    let harness = websocket_harness_with_options(&server, false, false, false, true).await;
+    let mut client_session = harness.client.new_session();
+    client_session
+        .prewarm_websocket(&harness.otel_manager, &harness.model_info, None)
+        .await
+        .expect("websocket prewarm failed");
+
+    // Prewarm should only perform the handshake, not send response.create.
+    assert_eq!(server.handshakes().len(), 1);
+    assert_eq!(server.single_connection().len(), 0);
+
+    let prompt = prompt_with_input(vec![message_item("hello")]);
+    stream_until_complete(&mut client_session, &harness, &prompt).await;
+
+    assert_eq!(server.handshakes().len(), 1);
+    assert_eq!(server.single_connection().len(), 1);
+
+    server.shutdown().await;
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 #[traced_test]
 async fn responses_websocket_emits_websocket_telemetry_events() {
@@ -887,26 +917,32 @@ async fn websocket_harness_with_runtime_metrics(
     server: &WebSocketTestServer,
     runtime_metrics_enabled: bool,
 ) -> WebsocketTestHarness {
-    websocket_harness_with_options(server, runtime_metrics_enabled, false).await
+    websocket_harness_with_options(server, runtime_metrics_enabled, true, false, false).await
 }
 
 async fn websocket_harness_with_v2(
     server: &WebSocketTestServer,
     websocket_v2_enabled: bool,
 ) -> WebsocketTestHarness {
-    websocket_harness_with_options(server, false, websocket_v2_enabled).await
+    websocket_harness_with_options(server, false, true, websocket_v2_enabled, false).await
 }
 
 async fn websocket_harness_with_options(
     server: &WebSocketTestServer,
     runtime_metrics_enabled: bool,
+    websocket_enabled: bool,
     websocket_v2_enabled: bool,
+    prefer_websockets: bool,
 ) -> WebsocketTestHarness {
     let provider = websocket_provider(server);
     let codex_home = TempDir::new().unwrap();
     let mut config = load_default_config_for_test(&codex_home).await;
     config.model = Some(MODEL.to_string());
-    config.features.enable(Feature::ResponsesWebsockets);
+    if websocket_enabled {
+        config.features.enable(Feature::ResponsesWebsockets);
+    } else {
+        config.features.disable(Feature::ResponsesWebsockets);
+    }
     if runtime_metrics_enabled {
         config.features.enable(Feature::RuntimeMetrics);
     }
@@ -914,7 +950,8 @@ async fn websocket_harness_with_options(
         config.features.enable(Feature::ResponsesWebsocketsV2);
     }
     let config = Arc::new(config);
-    let model_info = ModelsManager::construct_model_info_offline(MODEL, &config);
+    let mut model_info = ModelsManager::construct_model_info_offline(MODEL, &config);
+    model_info.prefer_websockets = prefer_websockets;
     let conversation_id = ThreadId::new();
     let auth_manager = AuthManager::from_auth_for_testing(CodexAuth::from_api_key("Test API Key"));
     let exporter = InMemoryMetricExporter::default();
@@ -944,7 +981,7 @@ async fn websocket_harness_with_options(
         provider.clone(),
         SessionSource::Exec,
         config.model_verbosity,
-        true,
+        websocket_enabled,
         websocket_v2_enabled,
         false,
         runtime_metrics_enabled,