feat: add batches API with OpenAI compatibility

Add complete batches API implementation with protocol, providers, and tests:

Core Infrastructure:
- Add batches API protocol using OpenAI Batch types directly
- Add Api.batches enum value and protocol mapping in resolver
- Add OpenAI "batch" file purpose support
- Include proper error handling (ConflictError, ResourceNotFoundError)

Reference Provider:
- Add ReferenceBatchesImpl with full CRUD operations (create, retrieve, cancel, list)
- Implement background batch processing with configurable concurrency
- Add SQLite KVStore backend for persistence
- Support /v1/chat/completions endpoint with request validation

Comprehensive Test Suite:
- Add unit tests for provider implementation with validation
- Add integration tests for end-to-end batch processing workflows
- Add error handling tests for validation, malformed inputs, and edge cases

Configuration:
- Add max_concurrent_batches and max_concurrent_requests_per_batch options
- Add provider documentation with sample configurations

Test with -

```
$ uv run llama stack build --image-type venv --providers inference=YOU_PICK,files=inline::localfs,batches=inline::reference --run &
$ LLAMA_STACK_CONFIG=http://localhost:8321 uv run pytest tests/unit/providers/batches tests/integration/batches --text-model YOU_PICK
```

llama_stack/providers/registry/batches.py

@@ -0,0 +1,26 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+from llama_stack.providers.datatypes import Api, InlineProviderSpec, ProviderSpec
+
+
+def available_providers() -> list[ProviderSpec]:
+    return [
+        InlineProviderSpec(
+            api=Api.batches,
+            provider_type="inline::reference",
+            pip_packages=["openai"],
+            module="llama_stack.providers.inline.batches.reference",
+            config_class="llama_stack.providers.inline.batches.reference.config.ReferenceBatchesImplConfig",
+            api_dependencies=[
+                Api.inference,
+                Api.files,
+                Api.models,
+            ],
+            description="Reference implementation of batches API with KVStore persistence.",
+        ),
+    ]