Add Nexus worker service for server-to-worker commands (#708)

## Summary

Defines a Nexus service for server-to-worker communication, starting
with activity cancellation support.

## Design Decision

We chose a **generic command API** (`ExecuteCommandsRequest` with
`oneof` command types) instead of a cancel-specific API. This allows a
future optimization to batch multiple commands (cancel, pause, etc) in a
single request and deliver to a worker in one RPC.

## Files

- `temporal/api/nexusservices/workerservice/v1/request_response.proto` -
request response definitions
- `nexus-rpc/temporal-proto-models-nexusrpc.yaml` - Nexus service
definition

## Related

- [Server PR](temporalio/temporal#9233)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82

nexus-rpc/temporal-proto-models-nexusrpc.yaml

@@ -0,0 +1,32 @@
+# yaml-language-server: $schema=https://raw.githubusercontent.com/nexus-rpc/nexus-rpc-gen/main/schemas/nexus-rpc-gen.json
+#
+# Nexus service definition for server-to-worker communication.
+# See request_response.proto for message definitions.
+#
+# Task queue format: /temporal-sys/worker-commands/{namespace}/{worker_grouping_key}
+
+nexusrpc: 1.0.0
+
+services:
+  temporal.api.nexusservices.workerservice.v1.WorkerService:
+    description: >
+      Internal Nexus service for server-to-worker communication.
+      Used by the Temporal server to send commands to workers.
+    operations:
+      ExecuteCommands:
+        description: Executes worker commands sent by the server.
+        input:
+          $goRef: "go.temporal.io/api/nexusservices/workerservice/v1.ExecuteCommandsRequest"
+          $javaRef: "io.temporal.api.nexusservices.workerservice.v1.ExecuteCommandsRequest"
+          $pythonRef: "temporalio.api.nexusservices.workerservice.v1.ExecuteCommandsRequest"
+          $typescriptRef: "@temporalio/api/nexusservices/workerservice/v1.ExecuteCommandsRequest"
+          $dotnetRef: "Temporalio.Api.Nexusservices.Workerservice.V1.ExecuteCommandsRequest"
+          $rubyRef: "Temporalio::Api::Nexusservices::Workerservice::V1::ExecuteCommandsRequest"
+        output:
+          $goRef: "go.temporal.io/api/nexusservices/workerservice/v1.ExecuteCommandsResponse"
+          $javaRef: "io.temporal.api.nexusservices.workerservice.v1.ExecuteCommandsResponse"
+          $pythonRef: "temporalio.api.nexusservices.workerservice.v1.ExecuteCommandsResponse"
+          $typescriptRef: "@temporalio/api/nexusservices/workerservice/v1.ExecuteCommandsResponse"
+          $dotnetRef: "Temporalio.Api.Nexusservices.Workerservice.V1.ExecuteCommandsResponse"
+          $rubyRef: "Temporalio::Api::Nexusservices::Workerservice::V1::ExecuteCommandsResponse"
+

temporal/api/nexusservices/workerservice/v1/request_response.proto

@@ -0,0 +1,27 @@
+syntax = "proto3";
+
+package temporal.api.nexusservices.workerservice.v1;
+
+option go_package = "go.temporal.io/api/nexusservices/workerservice/v1;workerservice";
+option java_package = "io.temporal.api.nexusservices.workerservice.v1";
+option java_multiple_files = true;
+option java_outer_classname = "RequestResponseProto";
+option ruby_package = "Temporalio::Api::Nexusservices::Workerservice::V1";
+option csharp_namespace = "Temporalio.Api.Nexusservices.Workerservice.V1";
+
+import "temporal/api/worker/v1/message.proto";
+
+// (--
+// Internal Nexus service for server-to-worker communication.
+// --)
+
+// Request payload for the "ExecuteCommands" Nexus operation.
+message ExecuteCommandsRequest {
+    repeated temporal.api.worker.v1.WorkerCommand commands = 1;
+}
+
+// Response payload for the "ExecuteCommands" Nexus operation.
+// The results list must be 1:1 with the commands list in the request (same size and order).
+message ExecuteCommandsResponse {
+    repeated temporal.api.worker.v1.WorkerCommandResult results = 1;
+}

temporal/api/worker/v1/message.proto

@@ -193,3 +193,27 @@ message StorageDriverInfo {
   // The type of the driver, required.
   string type = 1;
 }
+
+// A command sent from the server to a worker.
+message WorkerCommand {
+  oneof type {
+    CancelActivityCommand cancel_activity = 1;
+  }
+}
+
+// Cancel an activity if it is still running. Otherwise, do nothing.
+message CancelActivityCommand {
+  bytes task_token = 1;
+}
+
+// The result of executing a WorkerCommand.
+message WorkerCommandResult {
+  oneof type {
+    CancelActivityResult cancel_activity = 1;
+  }
+}
+
+// Result of a CancelActivityCommand.
+// Treat both successful cancellation and no-op (activity is no longer running) as success.
+message CancelActivityResult {
+}