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/codex-api/tests/models_integration.rs

@@ -89,6 +89,7 @@ async fn models_client_hits_models_endpoint() {
             experimental_supported_tools: Vec::new(),
             input_modalities: default_input_modalities(),
             prefer_websockets: false,
+            used_fallback_model_metadata: false,
         }],
     };