chore: import upstream snapshot with attribution

contributing/samples/workflows/nested_workflow/agent.py

@@ -0,0 +1,92 @@
+# 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.
+
+import re
+
+from google.adk import Agent
+from google.adk import Event
+from google.adk import Workflow
+from google.adk.workflow import JoinNode
+
+
+def process_input(node_input: str):
+  """Validates the input is a valid 4-digit year."""
+  match = re.search(r"\b\d{4}\b", node_input)
+  if not match:
+    yield Event(message="Please provide a valid 4-digit year (e.g., 1955).")
+    raise ValueError("Invalid year format.")
+
+  yield Event(state={"year": match.group(0)})
+
+
+find_name = Agent(
+    name="find_name",
+    instruction="""
+    Find the name of one famous person who was born in this year: {year}.
+    Return ONLY their name, nothing else.
+    """,
+)
+
+
+generate_bio = Agent(
+    name="generate_bio",
+    instruction="""
+    Write a short, engaging 3-sentence biography for the specified person.
+    """,
+)
+
+
+# Sub-workflow that acts as a single node in the parent workflow
+find_famous_person = Workflow(
+    name="find_famous_person",
+    edges=[("START", find_name, generate_bio)],
+)
+
+
+find_historical_event = Agent(
+    name="find_historical_event",
+    instruction="""
+    Describe one highly significant historical event that occurred in this year: {year}.
+    Keep the description to 2 sentences.
+    """,
+)
+
+join_for_aggregation = JoinNode(name="join_for_aggregation")
+
+
+def aggregate_results(node_input: dict[str, str], year: str):
+  """Combines outputs from parallel branches found in context state."""
+
+  combined_message = (
+      f"# Year: {year}\n\n"
+      "## Famous Person Bio:\n\n"
+      f"{node_input['find_famous_person']}\n\n"
+      "## Historical Event:\n\n"
+      f"{node_input['find_historical_event']}"
+  )
+  yield Event(message=combined_message)
+
+
+root_agent = Workflow(
+    name="root_agent",
+    edges=[
+        (
+            "START",
+            process_input,
+            (find_famous_person, find_historical_event),
+            join_for_aggregation,
+            aggregate_results,
+        ),
+    ],
+)