Add OnStopped method to dispose of _linkedTokenSource

This commit introduces the OnStopped method in the WorkflowInstanceImpl class to properly dispose of _linkedTokenSource. This change ensures that resources are appropriately cleaned up when the actor is stopped, preventing potential memory leaks.

src/modules/Elsa.Workflows.Runtime.ProtoActor/Actors/WorkflowInstanceImpl.cs

@@ -47,6 +47,12 @@ internal class WorkflowInstanceImpl(
         return Task.CompletedTask;
     }
 
+    public override Task OnStopped()
+    {
+        _linkedTokenSource.Dispose();
+        return Task.CompletedTask;
+    }
+
     public override Task<ProtoCreateWorkflowInstanceResponse> Create(ProtoCreateWorkflowInstanceRequest request) => throw new NotImplementedException();
 
     public override Task Create(ProtoCreateWorkflowInstanceRequest request, Action<ProtoCreateWorkflowInstanceResponse> respond, Action<string> onError)