fix: harden storage semantics (#4118)

Fixes issues in the storage system by guaranteeing immediate durability
for responses and ensuring background writers stay alive. Three related
fixes:

* Responses to the OpenAI-compatible API now write directly to
Postgres/SQLite inside the request instead of detouring through an async
queue that might never drain; this restores the expected
read-after-write behavior and removes the "response not found" races
reported by users.

* The access-control shim was stamping owner_principal/access_attributes
as SQL NULL, which Postgres interprets as non-public rows; fixing it to
use the empty-string/JSON-null pattern means conversations and responses
stored without an authenticated user stay queryable (matching SQLite).

* The inference-store queue remains for batching, but its worker tasks
now start lazily on the live event loop so server startup doesn't cancel
them—writes keep flowing even when the stack is launched via llama stack
run.

Closes #4115

Added a matrix entry to test our "base" suite against Postgres as the
store.

llama_stack/providers/utils/inference/inference_store.py

@@ -66,13 +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()))
-        else:
-            logger.info("Write queue disabled for SQLite to avoid concurrency issues")
-
     async def shutdown(self) -> None:
         if not self._worker_tasks:
             return
@@ -93,10 +86,29 @@ class InferenceStore:
         if self.enable_write_queue and self._queue is not None:
             await self._queue.join()
 
+    async def _ensure_workers_started(self) -> None:
+        """Ensure the async write queue workers run on the current loop."""
+        if not self.enable_write_queue:
+            return
+
+        if self._queue is None:
+            self._queue = asyncio.Queue(maxsize=self._max_write_queue_size)
+            logger.debug(
+                f"Inference store write queue created with max size {self._max_write_queue_size} "
+                f"and {self._num_writers} writers"
+            )
+
+        if not self._worker_tasks:
+            loop = asyncio.get_running_loop()
+            for _ in range(self._num_writers):
+                task = loop.create_task(self._worker_loop())
+                self._worker_tasks.append(task)
+
     async def store_chat_completion(
         self, chat_completion: OpenAIChatCompletion, input_messages: list[OpenAIMessageParam]
     ) -> None:
         if self.enable_write_queue:
+            await self._ensure_workers_started()
             if self._queue is None:
                 raise ValueError("Inference store is not initialized")
             try: