Add worker_instance_key to RespondWorkflowTaskCompletedRequest

This enables the server to track which worker is executing eager-dispatched
activities, allowing activity cancellation to be routed correctly.

Author: rkannan82

temporal/api/workflowservice/v1/request_response.proto

@@ -376,6 +376,9 @@ message RespondWorkflowTaskCompletedRequest {
     temporal.api.enums.v1.VersioningBehavior versioning_behavior = 16;
     // Worker deployment options that user has set in the worker.
     temporal.api.deployment.v1.WorkerDeploymentOptions deployment_options = 17;
+    // A unique key for this worker instance, used for tracking worker lifecycle.
+    // This is guaranteed to be unique, whereas identity is not guaranteed to be unique.
+    string worker_instance_key = 18;
 
     // SDK capability details.
     message Capabilities {