Revert support for the Cursor (SDK) provider

[DKLokeswaran]

With the built-in Cursor provider there's no way to revert. Hovering a user message never shows
"Revert to this message" — not the newest one, not older ones — and there's no checkpoint rollback
control either. Cursor is the only provider I use where this is missing, and nothing in the UI
explains why.

With Codex and Claude I can undo a turn that went the wrong way, fix the prompt, and keep working in
the same thread. I want the same thing with Cursor: pick an earlier message, go back to it, and
continue the thread from there.

Background and triage are on #17093.