Rename BaseTask → BaseWorker and reserve "task" for asyncio

Replaces every "task" identifier that referred to the BaseTask
abstraction with "worker". Asyncio task plumbing (asyncio.Task,
BaseTaskManager, TaskManager, create_task, cancel_task, etc.) stays
untouched. Highlights:

- Classes: BaseTask → BaseWorker, PipelineTask → PipelineWorker,
  LLMTask → LLMWorker, LLMContextTask → LLMContextWorker, TaskBus →
  WorkerBus, TaskRegistry → WorkerRegistry, TaskActivationArgs →
  WorkerActivationArgs, TaskReadyData → WorkerReadyData,
  TaskRegistryEntry → WorkerRegistryEntry, TaskObserver →
  WorkerObserver, all Bus*TaskMessage → Bus*WorkerMessage,
  BusAddTaskMessage.task field → worker, BusWorkerRegistryMessage.tasks
  field → workers.
- Methods/decorators: activate_task → activate_worker, deactivate_task
  → deactivate_worker, add_task → add_worker, watch_task →
  watch_worker, @task_ready → @worker_ready, setup_pipeline_task hook
  → setup_pipeline_worker.
- Params/fields: FrameProcessorSetup.pipeline_task and
  FunctionCallParams.pipeline_task → pipeline_worker. Parameter names
  like task_name → worker_name; spawn/run accept worker:.
- Files: pipeline/base_task.py → base_worker.py, pipeline/task.py →
  worker.py (plus a re-export shim at pipeline/task.py),
  task_observer.py → worker_observer.py, task_ready_decorator.py →
  worker_ready_decorator.py, pipecat.tasks → pipecat.workers,
  llm_task.py → llm_worker.py, llm_context_task.py →
  llm_context_worker.py, examples/multi-task → examples/multi-worker.

Back-compat:
- PipelineTask kept as a deprecated subclass of PipelineWorker that
  warns on construction.
- pipecat.pipeline.task re-exports PipelineWorker/PipelineTask/etc. so
  existing user imports keep working.
- FrameProcessor.pipeline_task kept as a deprecated property that
  forwards to pipeline_worker.

Local variables in examples that hold a worker (task = PipelineTask(...))
are renamed to worker = PipelineWorker(...). Asyncio-task locals
(runner_task, etc.) are preserved.

tests/test_pgmq_bus.py

@@ -12,7 +12,7 @@ from dataclasses import dataclass, field
 from datetime import datetime
 
 from pipecat.bus import (
-    BusAddTaskMessage,
+    BusAddWorkerMessage,
     BusDataMessage,
     BusEndMessage,
     BusFrameMessage,
@@ -21,7 +21,7 @@ from pipecat.bus import (
 )
 from pipecat.bus.serializers import JSONMessageSerializer
 from pipecat.frames.frames import TextFrame
-from pipecat.pipeline.base_task import BaseTask
+from pipecat.pipeline.base_worker import BaseWorker
 from pipecat.processors.frame_processor import FrameDirection
 from pipecat.utils.asyncio.task_manager import TaskManager, TaskManagerParams
 
@@ -171,8 +171,8 @@ class TestPgmqBus(unittest.IsolatedAsyncioTestCase):
         received = []
         await self.bus.subscribe(_make_sub(received))
 
-        task = BaseTask("test")
-        msg = BusAddTaskMessage(source="parent", task=task)
+        worker = BaseWorker("test")
+        msg = BusAddWorkerMessage(source="parent", worker=worker)
         await self.bus.send(msg)
 
         await asyncio.sleep(0.05)
@@ -181,8 +181,8 @@ class TestPgmqBus(unittest.IsolatedAsyncioTestCase):
         self.assertEqual(len(self.pgmq.sent), 0)
         # But delivered locally
         self.assertEqual(len(received), 1)
-        self.assertIsInstance(received[0], BusAddTaskMessage)
-        self.assertIs(received[0].task, task)
+        self.assertIsInstance(received[0], BusAddWorkerMessage)
+        self.assertIs(received[0].worker, worker)
 
     async def test_round_trip_via_subscriber(self):
         """Messages published are received by subscribers."""
@@ -367,7 +367,7 @@ class TestPgmqBusEdgeCases(unittest.IsolatedAsyncioTestCase):
         await bus.stop()
 
     async def test_stop_cleans_up(self):
-        """stop() cancels the reader task and drops the queue."""
+        """stop() cancels the reader worker and drops the queue."""
         pgmq = FakePgmq()
         bus = PgmqBus(pgmq=pgmq, channel="cleanup_test", poll_interval_ms=10, max_poll_seconds=1)
         tm = TaskManager()