## What changed
- Add the canonical agent path as `agent_name` in Responses API turn metadata.
- Fall back to `/root` when a session has no canonical agent path.
- Reserve `agent_name` from client metadata overrides and omit it from MCP request metadata.
## Testing
- Cover root, named subagent, legacy subagent, MCP, and client metadata merge cases.
GitOrigin-RevId: a9adcc079a51beae7d621aba24bcd759ceaa408e