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

docs/_static/llama-stack-spec.html

@@ -14767,7 +14767,8 @@
             "OpenAIFilePurpose": {
                 "type": "string",
                 "enum": [
-                    "assistants"
+                    "assistants",
+                    "batch"
                 ],
                 "title": "OpenAIFilePurpose",
                 "description": "Valid purpose values for OpenAI Files API."
@@ -14844,7 +14845,8 @@
                     "purpose": {
                         "type": "string",
                         "enum": [
-                            "assistants"
+                            "assistants",
+                            "batch"
                         ],
                         "description": "The intended purpose of the file"
                     }

docs/_static/llama-stack-spec.yaml

@@ -10951,6 +10951,7 @@ components:
       type: string
       enum:
         - assistants
+        - batch
       title: OpenAIFilePurpose
       description: >-
         Valid purpose values for OpenAI Files API.
@@ -11019,6 +11020,7 @@ components:
           type: string
           enum:
             - assistants
+            - batch
           description: The intended purpose of the file
       additionalProperties: false
       required: