chore: import upstream snapshot with attribution

contributing/samples/workflows/loop_config/README.md

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+# Workflow Loop Config Sample
+
+## Overview
+
+This sample demonstrates how to define a workflow with a feedback loop using a YAML configuration file. It mirrors the `workflow_samples/loop` sample, but uses YAML to define the workflow structure instead of Python.
+
+## Sample Inputs
+
+- `Python programming`
+
+- `Baking cookies`
+
+## Graph
+
+```mermaid
+graph TD
+    START --> process_input[process_input]
+    process_input --> generate_headline[generate_headline.yaml]
+    generate_headline --> evaluate_headline[evaluate_headline.yaml]
+    evaluate_headline --> route_headline[route_headline]
+    route_headline -->|unrelated| generate_headline
+```
+
+## How To
+
+This sample uses some special syntax in `root_agent.yaml` to support dynamic resolution and graph construction:
+
+### 1. `_code` Suffix
+
+Fields ending with `_code` (like `output_schema_code` in `evaluate_headline.yaml`) tell the ADK YAML mapper to resolve the value as a Python code reference rather than treating it as a plain string.
+
+- If it starts with `.`, it resolves relative to the current agent directory's Python package path.
+- Example: `output_schema_code: .agent.Feedback` resolves to the `Feedback` Pydantic model in `agent.py` in the same directory.
+
+### 2. Function References in Edges
+
+If a string in the edge list does not end with `.yaml` and is not `'START'`, it is treated as a function reference.
+
+- If it starts with `.`, it resolves relative to the current agent directory's Python package path.
+- Example: `.agent.process_input` resolves to the `process_input` function in `agent.py`.
+- It automatically creates a `FunctionNode` with the function's name as the node name.
+
+### 3. External Agent Files
+
+Agents can be defined in their own YAML files and referenced by filename in the edges list.
+
+- Example: `generate_headline.yaml` references the agent defined in that file.
+- The mapper caches resolved nodes by their string value, so using the same filename in multiple edges correctly reuses the same agent instance, preserving the graph structure (e.g. for loops).

contributing/samples/workflows/loop_config/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 typing import Literal
+
+from google.adk import Event
+from pydantic import BaseModel
+from pydantic import Field
+
+
+class Feedback(BaseModel):
+  grade: Literal["tech-related", "unrelated"] = Field(
+      description=(
+          "Decide if the headline is related to technology or software"
+          " engineering."
+      )
+  )
+  feedback: str = Field(
+      description=(
+          "If the headline is unrelated to technology, provide feedback on how"
+          " to make it more tech-focused."
+      )
+  )
+
+
+def process_input(node_input: str):
+  """Puts user input in the state."""
+  return Event(state={"topic": node_input})
+
+
+def route_headline(node_input: Feedback):
+  return Event(route=node_input.grade)

contributing/samples/workflows/loop_config/evaluate_headline.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.
+
+agent_class: LlmAgent
+name: evaluate_headline
+instruction: |
+  Grade whether the headline is related to technology or software engineering.
+output_schema_code: .agent.Feedback
+output_key: feedback

contributing/samples/workflows/loop_config/generate_headline.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.
+
+agent_class: LlmAgent
+name: generate_headline
+instruction: |
+  Write a headline about the topic "{topic}".
+  If feedback is provided, take it into account.
+  The feedback: {feedback?}

contributing/samples/workflows/loop_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.
+
+agent_class: Workflow
+name: loop_workflow
+edges:
+  - - START
+    - .agent.process_input
+    - generate_headline.yaml
+    - evaluate_headline.yaml
+    - .agent.route_headline
+  - - .agent.route_headline
+    - unrelated: generate_headline.yaml