add tests for initial prompt feature

> +1, I'm confused on how to test this.

With the current implementation, I think you'd need to test the `StartSnapshotLoop()` function of the server but pass in both an EventEmitter and a `Conversation` that you control in the test.

_Originally posted by @johnstcn in https://github.com/coder/agentapi/issues/112#issuecomment-3364720407_
            