chore: import upstream snapshot with attribution

contributing/samples/multi_agent/multi_agent_seq_config/README.md

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+# Config-based Agent Sample - Sequential Workflow
+
+A multi-agent setup with a sequential workflow.
+
+The whole process is:
+
+1. An agent backed by a cheap and fast model to write initial version.
+1. An agent backed by a smarter and a little more expensive to review the code.
+1. A final agent backed by the smartest and slowest model to write the final revision.
+
+Sample queries:
+
+> Write a quicksort method in python

contributing/samples/multi_agent/multi_agent_seq_config/root_agent.yaml

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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.
+
+# yaml-language-server: $schema=https://raw.githubusercontent.com/google/adk-python/refs/heads/main/src/google/adk/agents/config_schemas/AgentConfig.json
+agent_class: SequentialAgent
+name: CodePipelineAgent
+description: Executes a sequence of code writing, reviewing, and refactoring.
+sub_agents:
+  - config_path: sub_agents/code_writer_agent.yaml
+  - config_path: sub_agents/code_reviewer_agent.yaml
+  - config_path: sub_agents/code_refactorer_agent.yaml

contributing/samples/multi_agent/multi_agent_seq_config/sub_agents/code_refactorer_agent.yaml

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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.
+
+# yaml-language-server: $schema=https://raw.githubusercontent.com/google/adk-python/refs/heads/main/src/google/adk/agents/config_schemas/AgentConfig.json
+agent_class: LlmAgent
+name: CodeRefactorerAgent
+model: gemini-2.5-pro
+description: Refactors code based on review comments.
+instruction: |
+  You are a Python Code Refactoring AI.
+  Your goal is to improve the given Python code based on the provided review comments.
+
+    **Original Code:**
+    ```python
+    {generated_code}
+    ```
+
+    **Review Comments:**
+    {review_comments}
+
+  **Task:**
+  Carefully apply the suggestions from the review comments to refactor the original code.
+  If the review comments state "No major issues found," return the original code unchanged.
+  Ensure the final code is complete, functional, and includes necessary imports and docstrings.
+
+  **Output:**
+  Output *only* the final, refactored Python code block, enclosed in triple backticks (```python ... ```).
+  Do not add any other text before or after the code block.
+output_key: refactored_code

contributing/samples/multi_agent/multi_agent_seq_config/sub_agents/code_reviewer_agent.yaml

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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.
+
+# yaml-language-server: $schema=https://raw.githubusercontent.com/google/adk-python/refs/heads/main/src/google/adk/agents/config_schemas/AgentConfig.json
+agent_class: LlmAgent
+name: CodeReviewerAgent
+model: gemini-2.5-flash
+description: Reviews code and provides feedback.
+instruction: |
+  You are an expert Python Code Reviewer.
+  Your task is to provide constructive feedback on the provided code.
+
+  **Code to Review:**
+  ```python
+  {generated_code}
+  ```
+
+  **Review Criteria:**
+  1.  **Correctness:** Does the code work as intended? Are there logic errors?
+  2.  **Readability:** Is the code clear and easy to understand? Follows PEP 8 style guidelines?
+  3.  **Efficiency:** Is the code reasonably efficient? Any obvious performance bottlenecks?
+  4.  **Edge Cases:** Does the code handle potential edge cases or invalid inputs gracefully?
+  5.  **Best Practices:** Does the code follow common Python best practices?
+
+  **Output:**
+  Provide your feedback as a concise, bulleted list. Focus on the most important points for improvement.
+  If the code is excellent and requires no changes, simply state: "No major issues found."
+  Output *only* the review comments or the "No major issues" statement.
+output_key: review_comments

contributing/samples/multi_agent/multi_agent_seq_config/sub_agents/code_writer_agent.yaml

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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.
+
+# yaml-language-server: $schema=https://raw.githubusercontent.com/google/adk-python/refs/heads/main/src/google/adk/agents/config_schemas/AgentConfig.json
+agent_class: LlmAgent
+name: CodeWriterAgent
+model: gemini-2.5-flash
+description: Writes initial Python code based on a specification.
+instruction: |
+  You are a Python Code Generator.
+  Based *only* on the user's request, write Python code that fulfills the requirement.
+  Output *only* the complete Python code block, enclosed in triple backticks (```python ... ```).
+  Do not add any other text before or after the code block.
+output_key: generated_code