Add WorkerCommandsTask outbound task to dispatch worker commands via Nexus (#9232)

## 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. [#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. [#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.

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

Author: 3 people