The `user_message_id` parameter on `AgentConnection::prompt` was
`Option<UserMessageId>`, but `NativeAgentConnection::prompt` immediately
called `.expect()` on it, crashing when the value was `None`.

Because the `expect` message said `UserMessageId` was required, this PR
makes the parameter not optional which makes the panic impossible.

This means we also need to unconditionally generate an ID in `send`,
where before we gated this on whether `truncate` returned `Some`.
Generating IDs unconditionally should be find, especially because the
prompt requires them.

Release Notes:

- N/A