[codex] add response proxy subagent header test (#16876)

This adds end-to-end coverage for `responses-api-proxy` request dumps
when Codex spawns a subagent and validates that the `x-codex-window-id`
and `x-openai-subagent` are properly set.

codex-rs/core/tests/suite/mod.rs

@@ -122,6 +122,7 @@ mod request_permissions;
 #[cfg(not(target_os = "windows"))]
 mod request_permissions_tool;
 mod request_user_input;
+mod responses_api_proxy_headers;
 mod resume;
 mod resume_warning;
 mod review;