Preserve model metadata origin through runtime resolution (#58361)

## What changed

Return `CatalogModel` from model lookups, keeping metadata together with its `Bundled`, `Provider`, `ExplicitConfig`, or `Synthetic` origin through config overrides and runtime snapshots. Replace `used_fallback_model_metadata` checks with explicit `Synthetic` origin checks.

Share the underlying `Arc<ModelInfo>` with extension stores and use copy-on-write for metadata changes. Keep origin tracking outside serialized model metadata.

## Testing

Extend coverage for origin retention across cache reloads, namespaced model lookups, catalog fallback, and config overrides. Verify that step snapshots and extension stores share the same model metadata allocation.

GitOrigin-RevId: bb6130c94a7a795aa168ee24664a1dd453579fba

codex-rs/codex-api/tests/models_integration.rs

@@ -157,7 +157,6 @@ async fn models_client_hits_models_endpoint() {
             effective_context_window_percent: 95,
             experimental_supported_tools: Vec::new(),
             input_modalities: default_input_modalities(),
-            used_fallback_model_metadata: false,
             supports_search_tool: false,
             supports_experimental_context: false,
             use_responses_lite: false,