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/tests/test_processor.py

@@ -88,6 +88,38 @@ def test_processor_preserves_runtime_failure_reason() -> None:
     asyncio.run(scenario())
 
 
+def test_processor_reports_cancelled_status_with_reason() -> None:
+    async def scenario() -> None:
+        reports: list[dict[str, object]] = []
+
+        class CancelledExecutor:
+            async def run(self, assignment):
+                return AssignmentExecutionResult(
+                    status="cancelled",
+                    failure_reason="cancellation requested by control plane",
+                    metadata={"runtime_status": "cancelled"},
+                )
+
+        class RecordingClient:
+            async def report_result(self, assignment, **kwargs):
+                reports.append(kwargs)
+                return TerminalResultResponse(status="recorded")
+
+        result = await AssignmentProcessor(
+            RecordingClient(),  # type: ignore[arg-type]
+            CancelledExecutor(),
+        ).process(_assignment())
+
+        assert result.report_status == "recorded"
+        assert reports[0] == {
+            "status": "cancelled",
+            "failure_reason": "cancellation requested by control plane",
+            "result": {"runtime_status": "cancelled"},
+        }
+
+    asyncio.run(scenario())
+
+
 def test_status_tracker_sees_started_then_finished_even_on_raise() -> None:
     async def scenario() -> None:
         tracker = AgentStatusTracker()