Accept a federated task delegated from a peer orchestrator.

Wire format::

{
  "sender": { ...AgentCard... },
  "task":   { "id": "...", "message": "...", "role": "..." }
}

Returns 202 with the local federated-task id and the remote task id that was offered. Validation errors return HTTP 409 so that the caller's retry policy treats them as terminal (the peer is reachable and authoritative, no point retrying with the same body).