Add configurable Responses API request metadata (#37895)

## What changed

- Add `responses_api_metadata` for product-owned key/value metadata included in
  every Responses API turn metadata payload, including parent and subagent
  requests.
- Limit the map to 16 entries, ASCII identifier keys of at most 64 bytes, and
  values of at most 128 bytes. Reject reserved Codex metadata keys and ignore
  this setting in project-local configuration.
- Give configured product metadata precedence over app-server client metadata
  while keeping it out of metadata sent to external MCP servers.

## Testing

- Cover reserved-key validation, metadata precedence, MCP isolation, and
  propagation to parent and subagent Responses API requests.

GitOrigin-RevId: a7be798294fde25145ab375a468321bb4e4a49f1

codex-rs/thread-manager-sample/src/main.rs

@@ -281,6 +281,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
         respect_system_proxy: false,
         apps_mcp_product_sku: None,
+        responses_api_metadata: BTreeMap::new(),
         realtime_audio: RealtimeAudioConfig::default(),
         experimental_realtime_ws_base_url: None,
         experimental_realtime_webrtc_call_base_url: None,