Ensure concurrent prompts are handled.

This basically implements the pending prompt queueing RFD. When
a prompt request is received while the agent is already generating,
we immediately forward the message to Claude and then wait until
it replays the message back to us. When it replays it, we respond to
the previous prompt request with "end_turn", which also singifies to
the client that the message is now being processed by the agent.

See agentclientprotocol/agent-client-protocol#484.

Author: SteffenDE