feat: add batches API with OpenAI compatibility (#3088)

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

llama_stack/core/resolver.py

@@ -8,6 +8,7 @@ import inspect
 from typing import Any
 
 from llama_stack.apis.agents import Agents
+from llama_stack.apis.batches import Batches
 from llama_stack.apis.benchmarks import Benchmarks
 from llama_stack.apis.datasetio import DatasetIO
 from llama_stack.apis.datasets import Datasets
@@ -75,6 +76,7 @@ def api_protocol_map(external_apis: dict[Api, ExternalApiSpec] | None = None) ->
         Api.agents: Agents,
         Api.inference: Inference,
         Api.inspect: Inspect,
+        Api.batches: Batches,
         Api.vector_io: VectorIO,
         Api.vector_dbs: VectorDBs,
         Api.models: Models,

llama_stack/core/server/server.py

@@ -32,6 +32,7 @@ from fastapi.responses import JSONResponse, StreamingResponse
 from openai import BadRequestError
 from pydantic import BaseModel, ValidationError
 
+from llama_stack.apis.common.errors import ConflictError, ResourceNotFoundError
 from llama_stack.apis.common.responses import PaginatedResponse
 from llama_stack.cli.utils import add_config_distro_args, get_config_from_args
 from llama_stack.core.access_control.access_control import AccessDeniedError
@@ -128,6 +129,10 @@ def translate_exception(exc: Exception) -> HTTPException | RequestValidationErro
                 ]
             },
         )
+    elif isinstance(exc, ConflictError):
+        return HTTPException(status_code=409, detail=str(exc))
+    elif isinstance(exc, ResourceNotFoundError):
+        return HTTPException(status_code=404, detail=str(exc))
     elif isinstance(exc, ValueError):
         return HTTPException(status_code=httpx.codes.BAD_REQUEST, detail=f"Invalid value: {str(exc)}")
     elif isinstance(exc, BadRequestError):