[tpp][codex] pass multi-agent metadata to MCP tools call - #27495

Summary

• add agent_path to Codex MCP request turn metadata
• add MCP-only has_spawned_subagent metadata, computed from persisted thread-spawn edges
• emit /root for root sessions and preserve thread-spawn subagent paths such as /root/worker
• keep agent_path and has_spawned_subagent out of the normal turn metadata header; they are only emitted in MCP request _meta
• cover root/subagent MCP metadata behavior and the spawned-subagent MCP field in tests

Notes

Search Service MCP receives Codex turn metadata through MCP request _meta["x-codex-turn-metadata"]. This change makes the current agent path visible there, so Search Service MCP can infer direct subagent usage with agent_path != "/root".

It also adds has_spawned_subagent, which is computed at MCP call time by checking the current thread ID against the persisted thread_spawn_edges state using list_thread_spawn_children. This counts both open and closed spawned subagents and avoids scanning conversation history.

Example metadata:

{
  "x-codex-turn-metadata": {
    "agent_path": "/root",
    "has_spawned_subagent": true
  }
}

Slack context: https://openai-corpws.slack.com/archives/C0AP7B5JY0K/p1781124325871739?thread_ts=1781123263.746089&cid=C0AP7B5JY0K

This does not change the standalone v1/alpha/search request body directly.

Testing

• cargo fmt --all from codex-rs
• cargo check -p codex-core
• cargo test -p codex-core mcp_tool_call_request_meta_includes_has_spawned_subagent
• cargo test -p codex-core turn_metadata_state_includes_agent_path_only_in_request_meta
• cargo test -p codex-core turn_metadata_state_ignores_client_reserved_metadata_before_start

just was not installed in this environment, so I used the underlying Rust toolchain directly.