Record actual Responses endpoints in tracing spans (#40906)

## Why

Responses-compatible requests can use routes other than `/responses`, but their
tracing spans reported the default route regardless of the selected endpoint.

## What changed

- Populate `api.path` from the selected `ResponsesEndpoint` for HTTP and
  WebSocket requests.
- Retain the endpoint on WebSocket connections so stream-request spans report
  the same route as connection spans.

GitOrigin-RevId: 349eca94b27d4cc3a1adde8b257313506bdaf0f4