chore: introduce write queue for inference_store (#3383)

# What does this PR do?
Adds a write worker queue for writes to inference store. This avoids
overwhelming request processing with slow inference writes.

## Test Plan

Benchmark:
```
cd /docs/source/distributions/k8s-benchmark
# start mock server
python openai-mock-server.py --port 8000
# start stack server
LLAMA_STACK_LOGGING="all=WARNING" uv run --with llama-stack python -m llama_stack.core.server.server docs/source/distributions/k8s-benchmark/stack_run_config.yaml
# run benchmark script
uv run python3 benchmark.py --duration 120 --concurrent 50 --base-url=http://localhost:8321/v1/openai/v1 --model=vllm-inference/meta-llama/Llama-3.2-3B-Instruct
```
## RPS from 21 -> 57

llama_stack/core/routers/__init__.py

@@ -78,7 +78,10 @@ async def get_auto_router_impl(
 
     # TODO: move pass configs to routers instead
     if api == Api.inference and run_config.inference_store:
-        inference_store = InferenceStore(run_config.inference_store, policy)
+        inference_store = InferenceStore(
+            config=run_config.inference_store,
+            policy=policy,
+        )
         await inference_store.initialize()
         api_to_dep_impl["store"] = inference_store
 

llama_stack/core/routers/inference.py

@@ -90,6 +90,11 @@ class InferenceRouter(Inference):
 
     async def shutdown(self) -> None:
         logger.debug("InferenceRouter.shutdown")
+        if self.store:
+            try:
+                await self.store.shutdown()
+            except Exception as e:
+                logger.warning(f"Error during InferenceStore shutdown: {e}")
 
     async def register_model(
         self,