chore: import upstream snapshot with attribution

tests/resources/agents/async-tools/tools/python/delayed_echo.py

@@ -0,0 +1,23 @@
+"""delayed_echo fixture tool (slow; exercises async dispatch ↔ auto-delivery)."""
+
+from __future__ import annotations
+
+import time
+
+from omnigent_client.tools import tool
+
+
+@tool
+def delayed_echo(label: str) -> str:
+    """
+    Sleep 2s, then echo ``label`` inside an unambiguous marker.
+
+    The delay makes the dispatch -> auto-delivery sequence observable as
+    distinct events; the marker is a distinctive substring so e2e
+    assertions like ``"ECHO_FROM_ASYNC[..." in final_text`` are unambiguous.
+
+    :param label: Text to echo back, e.g. ``"alpha"``.
+    :returns: ``f"ECHO_FROM_ASYNC[{label}]"``.
+    """
+    time.sleep(2)
+    return f"ECHO_FROM_ASYNC[{label}]"