fix: InferenceStore workers being cancelled on event loop change (#4373)

# What does this PR do?

The InferenceStore write queue workers were created during initialize()
which runs in the app factory's event loop. When uvicorn starts, it
creates its own event loop, causing the original worker tasks to be
cancelled. This resulted in chat completions being queued but never
written to the database.

Root cause: asyncio tasks are bound to the event loop they were created
in. When the event loop changes (app factory -> uvicorn server), tasks
from the old loop are cancelled. However, the task references remained
in self._worker_tasks, so _ensure_workers_started() thought workers
were still running.

The fix removes eager worker creation from initialize(). Workers must
not be started during app factory execution because that event loop is
temporary. Instead, _ensure_workers_started() creates workers lazily
during store_chat_completion(), which runs during request handling and
is guaranteed to be in uvicorn's event loop.

Additionally, _ensure_workers_started() now checks for active (non-done)
tasks rather than just checking if the list is non-empty. This makes
the code resilient to any future event loop transitions.

This follows the standard async Python pattern: defer background task
creation until the server is actually running, not during app init.

## Test Plan

Run the server with postgres or sqlite, I tested with postgres:

```
uv run llama stack run llama_stack/distributions/starter/run-with-postgres-store.yaml
```

Execute an inference call:

```
export INFERENCE_MODEL=gpt-4o-mini && curl -fsS http://127.0.0.1:8321/v1/chat/completions -H "Content-Type: application/json" -d "{\"model\": \"openai/$INFERENCE_MODEL\",\"messages\": [{\"role\": \"user\", \"content\": \"What color is grass?\"}], \"max_tokens\": 128, \"temperature\": 0.0}"
```

Check the DB being populated:

```
podman exec postgres psql -U llamastack -d llamastack -t -c "SELECT COUNT(*) FROM chat_completions;" 
    1
```

**AGAINST STABLE BRANCH 0.3.X SINCE MAIN DOES NOT HAVE THE ISSUE.**

---------

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/utils/inference/inference_store.py

@@ -66,14 +66,6 @@ class InferenceStore:
             },
         )
 
-        if self.enable_write_queue:
-            self._queue = asyncio.Queue(maxsize=self._max_write_queue_size)
-            for _ in range(self._num_writers):
-                self._worker_tasks.append(asyncio.create_task(self._worker_loop()))
-            logger.debug(
-                f"Inference store write queue enabled with {self._num_writers} writers, max queue size {self._max_write_queue_size}"
-            )
-
     async def shutdown(self) -> None:
         if not self._worker_tasks:
             return