chore(scripts): add reusable notebook_setup sparse-checkout helper (#14768)

Add a reusable, standard-library bootstrap utility in scripts/notebook-setup/
for tutorials and sample notebooks that depend on sibling Python modules when
opened standalone in Google Colab or Colab Enterprise.

- Sparse-clones only the target sample subdirectory into a managed cache folder,
  self-heals interrupted non-git directories, fetches and checks out FETCH_HEAD
  on persistent runtimes, installs requirements.txt, and reloads imported sibling
  sample modules.
- Preserves existing local developer and CI repository checkouts without running
  git fetch or git checkout -f.
- Includes unit tests in scripts/notebook-setup/tests/notebook_setup_test.py.

Author: iennae

scripts/notebook-setup/README.md

@@ -0,0 +1,36 @@
+# Notebook Setup Helper (`notebook_setup.py`)
+
+`notebook_setup.py` provides a standard-library bootstrap utility for notebooks in `python-docs-samples` that depend on sibling Python modules and `requirements.txt` files when opened standalone in Google Colab or Colab Enterprise.
+
+## Usage in a Notebook
+
+```python
+# @title Download sample files and install packages (~30 seconds)
+from pathlib import Path
+import sys
+import tempfile
+import urllib.request
+
+BRANCH = "main"
+SETUP_PATH = Path(tempfile.gettempdir()) / "notebook_setup.py"
+for parent in (Path.cwd(), *Path.cwd().parents):
+    candidate = parent / "scripts/notebook-setup/notebook_setup.py"
+    if candidate.is_file():
+        SETUP_PATH = candidate
+        break
+else:
+    if not SETUP_PATH.is_file():
+        url = (
+            "https://raw.githubusercontent.com/GoogleCloudPlatform/"
+            f"python-docs-samples/{BRANCH}/scripts/notebook-setup/notebook_setup.py"
+        )
+        with urllib.request.urlopen(url, timeout=30) as response:
+            SETUP_PATH.write_bytes(response.read())
+
+setup_dir = str(SETUP_PATH.parent.resolve())
+if setup_dir not in sys.path:
+    sys.path.insert(0, setup_dir)
+import notebook_setup
+
+notebook_setup.bootstrap("people-and-planet-ai/weather-forecasting", branch=BRANCH)
+```

scripts/notebook-setup/__init__.py

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+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.