feat: add batches API with OpenAI compatibility (with inference replay) (#3162)

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
```

addresses #3066

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

llama_stack/providers/inline/batches/reference/config.py

@@ -0,0 +1,40 @@
+# 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 pydantic import BaseModel, Field
+
+from llama_stack.providers.utils.kvstore.config import KVStoreConfig, SqliteKVStoreConfig
+
+
+class ReferenceBatchesImplConfig(BaseModel):
+    """Configuration for the Reference Batches implementation."""
+
+    kvstore: KVStoreConfig = Field(
+        description="Configuration for the key-value store backend.",
+    )
+
+    max_concurrent_batches: int = Field(
+        default=1,
+        description="Maximum number of concurrent batches to process simultaneously.",
+        ge=1,
+    )
+
+    max_concurrent_requests_per_batch: int = Field(
+        default=10,
+        description="Maximum number of concurrent requests to process per batch.",
+        ge=1,
+    )
+
+    # TODO: add a max requests per second rate limiter
+
+    @classmethod
+    def sample_run_config(cls, __distro_dir__: str) -> dict:
+        return {
+            "kvstore": SqliteKVStoreConfig.sample_run_config(
+                __distro_dir__=__distro_dir__,
+                db_name="batches.db",
+            ),
+        }