Make timestamp parameter optional in chat API and default to server-side "time()"

[oleksandr-nc]

Follow-up from #463

Right now newSession and newMessage require clients to pass a timestamp. This is error-prone (see #452 where a client sent milliseconds instead of seconds) and redundant - the server already uses time() for assistant responses in ChattyLLMTaskListener

The timestamp parameter should be made optional and default to time() server-side.