feat(host-agent): stop assignment execution collaboratively on cancellation

- LeaseGuard gains an is_cancellation convenience property
- ActiveAssignmentRunner marks the lease lost with a cancellation
  reason when a renewal response reports cancel_requested
- AssignmentExecutor threads stop_reason through to TaskRunner/
  WorkflowRunner and maps a cancellation-flavored stop to
  AssignmentExecutionResult.status = "cancelled" instead of "failed"
- AssignmentProcessor forwards a three-way done/cancelled/failed
  status when reporting the terminal result
- Add/extend tests across lease, assignment, processor, and client

tests/test_host_agent_progress_reporting.py

@@ -58,6 +58,7 @@ class _FakeExecutor:
         assignment: AssignmentModel,
         *,
         should_stop: Any | None = None,
+        stop_reason: Any | None = None,
     ) -> Any:
         from host_agent.assignment import AssignmentExecutionResult