Move relay helpers into exec-server test support (#40712)

## What changed

- Move the reusable WebSocket relay, registration, frame capture, and encryption assertion helpers into `codex-exec-server-test-support`.
- Re-export the helpers from the existing relay test module so its callers keep the same interface.
- Add the Cargo dependencies and Bazel proto compile data required by the shared support module.

GitOrigin-RevId: 0383149c79ef86cb5e64a09baec41332f9eec4c6