A step's tool set no longer empties when the step is cancelled. The shared
pipeline already settles unstarted calls as cancelled; keeping admission lets
those calls keep their finalizers, and a wrapper result completed at
cancellation now reports cancellation instead of UNKNOWN_TOOL.
exit_plan_mode checks for a missing Agent explicitly instead of asserting it.
Document the activation types on the tools subsystem page, the turn/end code
for a failed tool condition, the replacement and no-set consequences in the
upgrade guide, and the admission rule in the parallel tool-call note.
Yichen Jiang
47b6867566