Read model ETags from WebSocket metadata events (#38251)

## What changed

- Extract `x-models-etag` from `codex.response.metadata` events and emit it as a `ModelsEtag` response event.
- Stop reading or reporting the model ETag from WebSocket upgrade headers.

## Testing

- Update the WebSocket response-stream test to deliver the ETag through a metadata event and verify that it is emitted.

GitOrigin-RevId: 9b9cff3d8a0a914549aca62d8d316483a9a47f69

codex-rs/cli/src/doctor.rs

@@ -2409,10 +2409,6 @@ async fn websocket_reachability_check(
         Ok(Ok(probe)) => {
             details.push(format!("handshake result: HTTP {}", probe.status));
             details.push(format!("reasoning header: {}", probe.reasoning_included));
-            details.push(format!(
-                "models etag present: {}",
-                probe.models_etag_present
-            ));
             details.push(format!(
                 "server model present: {}",
                 probe.server_model_present