chore: import upstream snapshot with attribution

contributing/samples/multi_agent/three_layer_transfer/README.md

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+# Three Layer Transfer Sample
+
+## Overview
+
+This sample demonstrates a three-layer multi-agent system built with the ADK Python toolkit, showcasing structured double-round-trip hierarchical transfers.
+
+## Sample Inputs
+
+- `Hello, who are you?`
+- `Can you write a short story about a lost kitten?`
+- `Please translate it into Spanish.`
+- `Looks great! Let's return to the project coordinator.`
+
+## Graph
+
+```mermaid
+graph TD
+    root_agent[root_agent] --> writer_agent[writer_agent]
+    writer_agent --> translator_agent[translator_agent]
+```
+
+## How To
+
+This sample demonstrates:
+
+1. Root delegating to middle-child node `writer_agent`.
+1. `writer_agent` delegating to leaf-grandchild node `translator_agent`.
+1. Grandchild returning back to the parent `writer_agent`.
+1. Parent returning back to the root coordinator `root_agent`.

contributing/samples/multi_agent/three_layer_transfer/__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/multi_agent/three_layer_transfer/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 Agent
+
+# --- Leaf Agent (Grandchild) ---
+translator_agent = Agent(
+    name="translator_agent",
+    description="Translates text into different languages.",
+    instruction="""
+      You are a translator. Your job is to translate the text provided to you into the requested language.
+      Once the translation is complete, output the translated text, explain what you did, and then transfer back to the writer_agent.
+    """,
+)
+
+
+# --- Middle Agent (Child) ---
+writer_agent = Agent(
+    name="writer_agent",
+    description=(
+        "Writes stories, articles, or essays, and manages translation requests."
+    ),
+    instruction="""
+      You are a professional writer.
+      When asked to write something, perform the writing task and present the result to the user.
+      If the user asks to translate the written content into another language, transfer the task to the translator_agent.
+      If the user is satisfied and wants to return to the main coordinator, transfer back to the root_agent.
+    """,
+    sub_agents=[translator_agent],
+)
+
+
+# --- Root Agent (Parent) ---
+root_agent = Agent(
+    name="root_agent",
+    description=(
+        "Project coordinator that delegates writing and translation tasks."
+    ),
+    instruction="""
+      You are a project coordinator.
+      If the user wants to write a story, essay, or article, transfer the task to the writer_agent.
+      Answer general inquiries yourself, but delegate writing-related tasks.
+    """,
+    sub_agents=[writer_agent],
+)