feat: Add per-agent locking to send message

[mattzh72]

Description

This is a short-term fix for send_message. If multiple send_message calls are sent, due to some state in the interface, it errors. This prevents multiple send_message calls from executing at the same time for the same agent.

Testing

We write a test simulating concurrent send_message requests. Without the changes in this PR, it errors:

FAILED tests/test_client.py::test_send_message_parallel[client0] - Failed: Task 0 failed with exception: Failed to send message: {"detail":"Generator is inactive"}