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

@@ -41,6 +41,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         experimental_supported_tools: Vec::new(),
         input_modalities: default_input_modalities(),
         prefer_websockets: false,
+        used_fallback_model_metadata: false,
     }
 }