Allow branching message history

[ianmacartney]

Allow specifying a parentMessageId on thread creation. Behavior:

• recentMessages will pull from the parent thread if there aren't enough messages in the new thread. If the parent thread doesn't have enough, it will go to its parent, etc.
• search will not search in any parent thread, only in the current thread. This is mostly a side-effect of how the search indexes are implemented right now. To fetch from a specific parent, you could always fetch context messages from the desired thread, then feed those into the generateText call as a list of messages directly..

The workaround for this currently is to copy the old messages over manually

[julionav]

@ianmacartney i was thinking on this and I am not fully sure this is the expected behaviour from a fork. Yeah I know a regular fork does what you say, but reflecting the parent chat relationship in a chat/agentic app would be quite challenging, and I am not fully sure users expect the complex parent/child relationship when forking a chat.

[ianmacartney]

I'm starting to think that doing a full copy would be a nice convenience function. But not a new capability.
And then anything else can be done in a developer's own tables with custom logic to fetch message history from parent threads in whatever way makes sense.. eg you could make a table with a threadId and parent message(s) and do fetchMessageContext on the parent messages

[ianmacartney]

going to consider this a "wontfix" for now