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

tests/integration/batches/conftest.py

@@ -0,0 +1,122 @@
+# 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.
+
+"""Shared pytest fixtures for batch tests."""
+
+import json
+import time
+import warnings
+from contextlib import contextmanager
+from io import BytesIO
+
+import pytest
+
+from llama_stack.apis.files import OpenAIFilePurpose
+
+
+class BatchHelper:
+    """Helper class for creating and managing batch input files."""
+
+    def __init__(self, client):
+        """Initialize with either a batch_client or openai_client."""
+        self.client = client
+
+    @contextmanager
+    def create_file(self, content: str | list[dict], filename_prefix="batch_input"):
+        """Context manager for creating and cleaning up batch input files.
+
+        Args:
+            content: Either a list of batch request dictionaries or raw string content
+            filename_prefix: Prefix for the generated filename (or full filename if content is string)
+
+        Yields:
+            The uploaded file object
+        """
+        if isinstance(content, str):
+            # Handle raw string content (e.g., malformed JSONL, empty files)
+            file_content = content.encode("utf-8")
+        else:
+            # Handle list of batch request dictionaries
+            jsonl_content = "\n".join(json.dumps(req) for req in content)
+            file_content = jsonl_content.encode("utf-8")
+
+        filename = filename_prefix if filename_prefix.endswith(".jsonl") else f"{filename_prefix}.jsonl"
+
+        with BytesIO(file_content) as file_buffer:
+            file_buffer.name = filename
+            uploaded_file = self.client.files.create(file=file_buffer, purpose=OpenAIFilePurpose.BATCH)
+
+        try:
+            yield uploaded_file
+        finally:
+            try:
+                self.client.files.delete(uploaded_file.id)
+            except Exception:
+                warnings.warn(
+                    f"Failed to cleanup file {uploaded_file.id}: {uploaded_file.filename}",
+                    stacklevel=2,
+                )
+
+    def wait_for(
+        self,
+        batch_id: str,
+        max_wait_time: int = 60,
+        sleep_interval: int | None = None,
+        expected_statuses: set[str] | None = None,
+        timeout_action: str = "fail",
+    ):
+        """Wait for a batch to reach a terminal status.
+
+        Args:
+            batch_id: The batch ID to monitor
+            max_wait_time: Maximum time to wait in seconds (default: 60 seconds)
+            sleep_interval: Time to sleep between checks in seconds (default: 1/10th of max_wait_time, min 1s, max 15s)
+            expected_statuses: Set of expected terminal statuses (default: {"completed"})
+            timeout_action: Action on timeout - "fail" (pytest.fail) or "skip" (pytest.skip)
+
+        Returns:
+            The final batch object
+
+        Raises:
+            pytest.Failed: If batch reaches an unexpected status or timeout_action is "fail"
+            pytest.Skipped: If timeout_action is "skip" on timeout or unexpected status
+        """
+        if sleep_interval is None:
+            # Default to 1/10th of max_wait_time, with min 1s and max 15s
+            sleep_interval = max(1, min(15, max_wait_time // 10))
+
+        if expected_statuses is None:
+            expected_statuses = {"completed"}
+
+        terminal_statuses = {"completed", "failed", "cancelled", "expired"}
+        unexpected_statuses = terminal_statuses - expected_statuses
+
+        start_time = time.time()
+        while time.time() - start_time < max_wait_time:
+            current_batch = self.client.batches.retrieve(batch_id)
+
+            if current_batch.status in expected_statuses:
+                return current_batch
+            elif current_batch.status in unexpected_statuses:
+                error_msg = f"Batch reached unexpected status: {current_batch.status}"
+                if timeout_action == "skip":
+                    pytest.skip(error_msg)
+                else:
+                    pytest.fail(error_msg)
+
+            time.sleep(sleep_interval)
+
+        timeout_msg = f"Batch did not reach expected status {expected_statuses} within {max_wait_time} seconds"
+        if timeout_action == "skip":
+            pytest.skip(timeout_msg)
+        else:
+            pytest.fail(timeout_msg)
+
+
+@pytest.fixture
+def batch_helper(openai_client):
+    """Fixture that provides a BatchHelper instance for OpenAI client."""
+    return BatchHelper(openai_client)