Add WorkerCommandsTask outbound task to dispatch worker commands via Nexus

[rkannan82]

What changed?

New outbound task type (WorkerCommandsTask) that carries worker commands to be dispatched to workers via Nexus. Uses the generic WorkerCommand proto (not cancel-activity-specific), so this task type can carry any future command types.

Suggested review order: proto changes → worker_commands_task.go → task_generator.go → workflow_task_completed_handler.go

Key pieces:

• Proto: TASK_TYPE_WORKER_COMMANDS enum, WorkerCommandsTask in OutboundTaskInfo with repeated WorkerCommand.
• Task definition: worker_commands_task.go — implements outbound Task and HasDestination interfaces.
• Task creation (workflow_task_completed_handler.go, task_generator.go): When RequestCancelActivityTask is processed for a started activity whose worker has a control queue, collects a CancelActivityCommand with the activity's task token. Commands are batched by destination control queue and flushed as one WorkerCommandsTask per queue at the end of WFT processing.
• Serialization: task_serializers.go for persistence round-tripping.

Dispatch is a no-op here — handled in #9233. Gated by dynamic config EnableCancelActivityWorkerCommand (default: off).

Why?

To support proactive activity cancellation without waiting for heartbeat. This is the task creation leg of the flow.

1. [Store worker attributes needed by server to propagate nexus tasks to worker #9231] Store worker_control_task_queue in ActivityInfo at activity start.
2. [This PR] On RequestCancelActivityTask, batch commands by control queue into WorkerCommandsTask outbound tasks.
3. [Dispatch activity cancellation to worker using Nexus #9233] Dispatch each task as a Nexus ExecuteCommands operation to the worker, with a 3-attempt retry cap.
4. [SDK] Worker receives the cancel command and cancels the running activity.

Gated by dynamic config EnableCancelActivityWorkerCommand (default: off).

How did you test it?

Unit tests cover task generation, command batching (including multi-queue batching), task serialization round-tripping, and the feature-flag-off path.

[semgrep-managed-scans]

Semgrep found 1 missing-explicit-permissions finding:

• .github/workflows/promote-docker-image.yml
  • L26-80 - Triage

No explicit GITHUB_TOKEN permissions found at the workflow or job level. Add a permissions: block at the workflow root (applies to all jobs) or per job with least privilege (e.g., contents: read and only specific writes like pull-requests: write if needed).