chore: import upstream snapshot with attribution

tests/test_handoff_prompt.py

@@ -0,0 +1,12 @@
+from agents.extensions.handoff_prompt import (
+    RECOMMENDED_PROMPT_PREFIX,
+    prompt_with_handoff_instructions,
+)
+
+
+def test_prompt_with_handoff_instructions_includes_prefix() -> None:
+    prompt = "Handle the transfer smoothly."
+    result = prompt_with_handoff_instructions(prompt)
+
+    assert result.startswith(RECOMMENDED_PROMPT_PREFIX)
+    assert result.endswith(prompt)