chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/apis/batches/__init__.py

@@ -0,0 +1,9 @@
+# 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 .batches import Batches, BatchObject, ListBatchesResponse
+
+__all__ = ["Batches", "BatchObject", "ListBatchesResponse"]

src/llama_stack/apis/batches/batches.py

@@ -0,0 +1,100 @@
+# 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 typing import Literal, Protocol, runtime_checkable
+
+from pydantic import BaseModel, Field
+
+from llama_stack.apis.version import LLAMA_STACK_API_V1
+from llama_stack.schema_utils import json_schema_type, webmethod
+
+try:
+    from openai.types import Batch as BatchObject
+except ImportError as e:
+    raise ImportError("OpenAI package is required for batches API. Please install it with: pip install openai") from e
+
+
+@json_schema_type
+class ListBatchesResponse(BaseModel):
+    """Response containing a list of batch objects."""
+
+    object: Literal["list"] = "list"
+    data: list[BatchObject] = Field(..., description="List of batch objects")
+    first_id: str | None = Field(default=None, description="ID of the first batch in the list")
+    last_id: str | None = Field(default=None, description="ID of the last batch in the list")
+    has_more: bool = Field(default=False, description="Whether there are more batches available")
+
+
+@runtime_checkable
+class Batches(Protocol):
+    """
+    The Batches API enables efficient processing of multiple requests in a single operation,
+    particularly useful for processing large datasets, batch evaluation workflows, and
+    cost-effective inference at scale.
+
+    The API is designed to allow use of openai client libraries for seamless integration.
+
+    This API provides the following extensions:
+     - idempotent batch creation
+
+    Note: This API is currently under active development and may undergo changes.
+    """
+
+    @webmethod(route="/openai/v1/batches", method="POST", level=LLAMA_STACK_API_V1, deprecated=True)
+    @webmethod(route="/batches", method="POST", level=LLAMA_STACK_API_V1)
+    async def create_batch(
+        self,
+        input_file_id: str,
+        endpoint: str,
+        completion_window: Literal["24h"],
+        metadata: dict[str, str] | None = None,
+        idempotency_key: str | None = None,
+    ) -> BatchObject:
+        """Create a new batch for processing multiple API requests.
+
+        :param input_file_id: The ID of an uploaded file containing requests for the batch.
+        :param endpoint: The endpoint to be used for all requests in the batch.
+        :param completion_window: The time window within which the batch should be processed.
+        :param metadata: Optional metadata for the batch.
+        :param idempotency_key: Optional idempotency key. When provided, enables idempotent behavior.
+        :returns: The created batch object.
+        """
+        ...
+
+    @webmethod(route="/openai/v1/batches/{batch_id}", method="GET", level=LLAMA_STACK_API_V1, deprecated=True)
+    @webmethod(route="/batches/{batch_id}", method="GET", level=LLAMA_STACK_API_V1)
+    async def retrieve_batch(self, batch_id: str) -> BatchObject:
+        """Retrieve information about a specific batch.
+
+        :param batch_id: The ID of the batch to retrieve.
+        :returns: The batch object.
+        """
+        ...
+
+    @webmethod(route="/openai/v1/batches/{batch_id}/cancel", method="POST", level=LLAMA_STACK_API_V1, deprecated=True)
+    @webmethod(route="/batches/{batch_id}/cancel", method="POST", level=LLAMA_STACK_API_V1)
+    async def cancel_batch(self, batch_id: str) -> BatchObject:
+        """Cancel a batch that is in progress.
+
+        :param batch_id: The ID of the batch to cancel.
+        :returns: The updated batch object.
+        """
+        ...
+
+    @webmethod(route="/openai/v1/batches", method="GET", level=LLAMA_STACK_API_V1, deprecated=True)
+    @webmethod(route="/batches", method="GET", level=LLAMA_STACK_API_V1)
+    async def list_batches(
+        self,
+        after: str | None = None,
+        limit: int = 20,
+    ) -> ListBatchesResponse:
+        """List all batches for the current user.
+
+        :param after: A cursor for pagination; returns batches after this batch ID.
+        :param limit: Number of batches to return (default 20, max 100).
+        :returns: A list of batch objects.
+        """
+        ...