docs: move module docstring to top of handoff_filters.py (#2950)

Author: kuishou68

src/agents/extensions/handoff_filters.py

@@ -1,3 +1,5 @@
+"""Contains common handoff input filters, for convenience."""
+
 from __future__ import annotations
 
 from ..handoffs import (
@@ -21,8 +23,6 @@
     TResponseInputItem,
 )
 
-"""Contains common handoff input filters, for convenience. """
-
 __all__ = [
     "remove_all_tools",
     "nest_handoff_history",