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 .`

llama_stack/apis/datasetio/datasetio.py

@@ -1,59 +0,0 @@
-# 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 Any, Protocol, runtime_checkable
-
-from llama_stack.apis.common.responses import PaginatedResponse
-from llama_stack.apis.datasets import Dataset
-from llama_stack.apis.version import LLAMA_STACK_API_V1, LLAMA_STACK_API_V1BETA
-from llama_stack.schema_utils import webmethod
-
-
-class DatasetStore(Protocol):
-    def get_dataset(self, dataset_id: str) -> Dataset: ...
-
-
-@runtime_checkable
-class DatasetIO(Protocol):
-    # keeping for aligning with inference/safety, but this is not used
-    dataset_store: DatasetStore
-
-    @webmethod(route="/datasetio/iterrows/{dataset_id:path}", method="GET", deprecated=True, level=LLAMA_STACK_API_V1)
-    @webmethod(route="/datasetio/iterrows/{dataset_id:path}", method="GET", level=LLAMA_STACK_API_V1BETA)
-    async def iterrows(
-        self,
-        dataset_id: str,
-        start_index: int | None = None,
-        limit: int | None = None,
-    ) -> PaginatedResponse:
-        """Get a paginated list of rows from a dataset.
-
-        Uses offset-based pagination where:
-        - start_index: The starting index (0-based). If None, starts from beginning.
-        - limit: Number of items to return. If None or -1, returns all items.
-
-        The response includes:
-        - data: List of items for the current page.
-        - has_more: Whether there are more items available after this set.
-
-        :param dataset_id: The ID of the dataset to get the rows from.
-        :param start_index: Index into dataset for the first row to get. Get all rows if None.
-        :param limit: The number of rows to get.
-        :returns: A PaginatedResponse.
-        """
-        ...
-
-    @webmethod(
-        route="/datasetio/append-rows/{dataset_id:path}", method="POST", deprecated=True, level=LLAMA_STACK_API_V1
-    )
-    @webmethod(route="/datasetio/append-rows/{dataset_id:path}", method="POST", level=LLAMA_STACK_API_V1BETA)
-    async def append_rows(self, dataset_id: str, rows: list[dict[str, Any]]) -> None:
-        """Append rows to a dataset.
-
-        :param dataset_id: The ID of the dataset to append the rows to.
-        :param rows: The rows to append to the dataset.
-        """
-        ...