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

apps/device-host-agent/host_agent/lease.py

@@ -12,6 +12,7 @@ from cloud.internal_api.models import AssignmentModel
 from host_agent.assignment import AssignmentExecutionResult
 from host_agent.client import HostAgentAPIError, HostAgentClient, StaleLeaseError
 from host_agent.progress import TaskProgressSnapshot
+from runtime.task import is_cancellation_reason
 
 
 class InterruptibleAssignmentExecutor(Protocol):
@@ -20,6 +21,7 @@ class InterruptibleAssignmentExecutor(Protocol):
         assignment: AssignmentModel,
         *,
         should_stop: Callable[[], bool] | None = None,
+        stop_reason: Callable[[], str | None] | None = None,
     ) -> AssignmentExecutionResult: ...
 
     def latest_progress(self) -> TaskProgressSnapshot | None: ...
@@ -36,6 +38,10 @@ class LeaseGuard:
         with self._lock:
             return self._reason
 
+    @property
+    def is_cancellation(self) -> bool:
+        return is_cancellation_reason(self.reason)
+
     def is_lost(self) -> bool:
         return self._lost.is_set()
 
@@ -69,6 +75,7 @@ class ActiveAssignmentRunner:
                 self.executor.execute,
                 assignment,
                 should_stop=lambda: guard.is_lost() or self._stop_requested.is_set(),
+                stop_reason=lambda: guard.reason,
             )
         )
         renewal = asyncio.create_task(
@@ -108,4 +115,7 @@ class ActiveAssignmentRunner:
                 guard.mark_lost("lease renewal failed after transport retries")
                 return
             else:
+                if response.cancel_requested:
+                    guard.mark_lost("cancellation requested by control plane")
+                    return
                 lease_expires_at = response.lease_expires_at