Add a regression test that actually runs the CancelledError branch of
AgentLoop._dispatch end-to-end and asserts the in-flight checkpoint is
materialized into session.messages before the cancellation unwinds.
The three existing tests call _restore_runtime_checkpoint directly, so
they pass even if the cancel-time restore is ever removed from
_dispatch. This new test is the one that actually locks the fix in
place.
Made-with: Cursor