fix: show user warning when using default fallback metadata (#11690)

### What
It's currently unclear when the harness falls back to the default,
generic `ModelInfo`. This happens when the `remote_models` feature is
disabled or the model is truly unknown, and can lead to bad performance
and issues in the harness.

Add a user-facing warning when this happens so they are aware when their
setup is broken.

### Tests
Added tests, tested locally.

codex-rs/core/tests/suite/model_switching.rs

@@ -228,6 +228,7 @@ async fn model_change_from_image_to_text_strips_prior_image_content() -> Result<
         supported_in_api: true,
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
         priority: 1,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
@@ -385,6 +386,7 @@ async fn model_switch_to_smaller_model_updates_token_context_window() -> Result<
         supported_in_api: true,
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
         priority: 1,
         upgrade: None,
         base_instructions: "base instructions".to_string(),

codex-rs/core/tests/suite/models_cache_ttl.rs

@@ -352,5 +352,6 @@ fn test_remote_model(slug: &str, priority: i32) -> ModelInfo {
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
     }
 }

codex-rs/core/tests/suite/personality.rs

@@ -616,6 +616,7 @@ async fn ignores_remote_personality_if_remote_models_disabled() -> anyhow::Resul
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
     };
 
     let _models_mock = mount_models_once(
@@ -733,6 +734,7 @@ async fn remote_model_friendly_personality_instructions_with_feature() -> anyhow
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
     };
 
     let _models_mock = mount_models_once(
@@ -845,6 +847,7 @@ async fn user_turn_personality_remote_model_template_includes_update_message() -
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
     };
 
     let _models_mock = mount_models_once(

codex-rs/core/tests/suite/remote_models.rs

@@ -225,6 +225,7 @@ async fn remote_models_remote_model_uses_unified_exec() -> Result<()> {
         supported_in_api: true,
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
         priority: 1,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
@@ -464,6 +465,7 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
         supported_in_api: true,
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
         priority: 1,
         upgrade: None,
         base_instructions: remote_base.to_string(),
@@ -948,6 +950,7 @@ fn test_remote_model_with_policy(
         supported_in_api: true,
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
         priority,
         upgrade: None,
         base_instructions: "base instructions".to_string(),

codex-rs/core/tests/suite/resume_warning.rs

@@ -74,7 +74,14 @@ async fn emits_warning_when_resumed_model_differs() {
         .expect("resume conversation");
 
     // Assert: a Warning event is emitted describing the model mismatch.
-    let warning = wait_for_event(&conversation, |ev| matches!(ev, EventMsg::Warning(_))).await;
+    let warning = wait_for_event(&conversation, |ev| {
+        matches!(
+            ev,
+            EventMsg::Warning(WarningEvent { message })
+                if message.contains("previous-model") && message.contains("current-model")
+        )
+    })
+    .await;
     let EventMsg::Warning(WarningEvent { message }) = warning else {
         panic!("expected warning event");
     };

codex-rs/core/tests/suite/rmcp_client.rs

@@ -410,6 +410,7 @@ async fn stdio_image_responses_are_sanitized_for_text_only_model() -> anyhow::Re
                 experimental_supported_tools: Vec::new(),
                 input_modalities: vec![InputModality::Text],
                 prefer_websockets: false,
+                used_fallback_model_metadata: false,
             }],
         },
     )

codex-rs/core/tests/suite/view_image.rs

@@ -671,6 +671,7 @@ async fn view_image_tool_returns_unsupported_message_for_text_only_model() -> an
         supported_in_api: true,
         input_modalities: vec![InputModality::Text],
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
         priority: 1,
         upgrade: None,
         base_instructions: "base instructions".to_string(),