Add a health endpoint to the code-mode gRPC listener (#38806)

## What changed

- Serve `GET /healthz` with a `200 OK` response over HTTP/1.1 and HTTP/2.
- Continue requiring HTTP/2 for all other requests so gRPC methods are not exposed over HTTP/1.1.

## Testing

- Add TCP listener integration coverage for HTTP/1.1 and HTTP/2 health checks and rejection of HTTP/1.1 gRPC requests.

GitOrigin-RevId: ae7bbf56323fbc76769375a6d8e90653e8adc860