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

scripts/provider_codegen.py

@@ -18,6 +18,23 @@ from llama_stack.core.distribution import get_provider_registry
 REPO_ROOT = Path(__file__).parent.parent
 
 
+def get_api_docstring(api_name: str) -> str | None:
+    """Extract docstring from the API protocol class."""
+    try:
+        # Import the API module dynamically
+        api_module = __import__(f"llama_stack.apis.{api_name}", fromlist=[api_name.title()])
+
+        # Get the main protocol class (usually capitalized API name)
+        protocol_class_name = api_name.title()
+        if hasattr(api_module, protocol_class_name):
+            protocol_class = getattr(api_module, protocol_class_name)
+            return protocol_class.__doc__
+    except (ImportError, AttributeError):
+        pass
+
+    return None
+
+
 class ChangedPathTracker:
     """Track a list of paths we may have changed."""
 
@@ -261,6 +278,11 @@ def process_provider_registry(progress, change_tracker: ChangedPathTracker) -> N
             index_content.append(f"# {api_name.title()}\n")
             index_content.append("## Overview\n")
 
+            api_docstring = get_api_docstring(api_name)
+            if api_docstring:
+                cleaned_docstring = api_docstring.strip()
+                index_content.append(f"{cleaned_docstring}\n")
+
             index_content.append(
                 f"This section contains documentation for all available providers for the **{api_name}** API.\n"
             )