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/providers/utils/datasetio/__init__.py

@@ -0,0 +1,5 @@
+# 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.

src/llama_stack/providers/utils/datasetio/url_utils.py

@@ -0,0 +1,47 @@
+# 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.
+
+import asyncio
+import base64
+import io
+from urllib.parse import unquote
+
+from llama_stack.providers.utils.memory.vector_store import parse_data_url
+
+
+async def get_dataframe_from_uri(uri: str):
+    import pandas
+
+    df = None
+    if uri.endswith(".csv"):
+        # Moving to its own thread to avoid io from blocking the eventloop
+        # This isn't ideal as it moves more then just the IO to a new thread
+        # but it is as close as we can easly get
+        df = await asyncio.to_thread(pandas.read_csv, uri)
+    elif uri.endswith(".xlsx"):
+        df = await asyncio.to_thread(pandas.read_excel, uri)
+    elif uri.startswith("data:"):
+        parts = parse_data_url(uri)
+        data = parts["data"]
+        if parts["is_base64"]:
+            data = base64.b64decode(data)
+        else:
+            data = unquote(data)
+            encoding = parts["encoding"] or "utf-8"
+            data = data.encode(encoding)
+
+        mime_type = parts["mimetype"]
+        mime_category = mime_type.split("/")[0]
+        data_bytes = io.BytesIO(data)
+
+        if mime_category == "text":
+            df = pandas.read_csv(data_bytes)
+        else:
+            df = pandas.read_excel(data_bytes)
+    else:
+        raise ValueError(f"Unsupported file type: {uri}")
+
+    return df