This is behind the beta flag. But some agents don't support a full load
flow, but rather just a resume from the current state of the session.
They don't emit notifications for past thread messages, which isn't
ideal, but the user can at least resume their work without completely
losing it.

We use it as a fallback if the agent doesn't support loading only, and
signal to the user that they won't be able to see previous messages.

Release Notes:

- N/A