fix(responses): preserve Codex client metadata

[Jeadie]

OpenAI Codex includes request metadata in v1/responses under client_metadata.

For example:

{
  "client_metadata": {
    "x-codex-installation-id": "...",
    "thread_id": "...",
    "session_id": "...",
    "x-codex-window-id": "...",
    "turn_id": "...",
    "x-codex-turn-metadata": "..."
  }
}

Preserve this metadata in the typed Responses request so gateways can forward it. Add a serde roundtrip test covering all Codex metadata fields.

This is backwards compatible: the field is optional, and requests that do not include Codex metadata are unchanged.