Make isolate re-use the same agent as long as the connection is open

[isidentical]

In the current implementation, we just receive a single callable, call it, and exit from the agent. But since each connection can actually execute multiple callables, it is really inefficient to re-start the agent process for no reason.

https://github.com/fal-ai/isolate/blob/6587d963e86d8b73c2683b828b058a562daf8a15/src/isolate/backends/connections/ipc/agent.py#L107-L137