chore: import upstream snapshot with attribution

contributing/samples/integrations/langchain_youtube_search_agent/README.md

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+# Langchain YouTube Search Agent
+
+This agent utilizes the Langchain YoutubeSearchTool to search Youtube Videos.
+You need to install the following dependencies:
+
+```python
+uv pip install youtube_search
+```

contributing/samples/integrations/langchain_youtube_search_agent/__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.
+
+from . import agent

contributing/samples/integrations/langchain_youtube_search_agent/agent.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.
+
+from google.adk.agents.llm_agent import LlmAgent
+from google.adk.tools.langchain_tool import LangchainTool
+from langchain_community.tools.youtube.search import YouTubeSearchTool
+
+# Instantiate the tool
+langchain_yt_tool = YouTubeSearchTool()
+
+# Wrap the tool in the LangchainTool class from ADK
+adk_yt_tool = LangchainTool(
+    tool=langchain_yt_tool,
+)
+
+root_agent = LlmAgent(
+    name="youtube_search_agent",
+    instruction="""
+    Ask customer to provide singer name, and the number of videos to search.
+    """,
+    description="Help customer to search for a video on Youtube.",
+    tools=[adk_yt_tool],
+    output_key="youtube_search_output",
+)

contributing/samples/integrations/langchain_youtube_search_agent/requirements.txt

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+youtube_search