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/datasetio/datasetio.py

@@ -0,0 +1,59 @@
+# 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.
+        """
+        ...