fix: remove duplicate CompactionItem from RunItem union (#2761)

Author: KanchiShimono

src/agents/items.py

@@ -609,7 +609,6 @@ def to_input_item(self) -> TResponseInputItem:
     HandoffOutputItem,
     ToolCallItem,
     ToolCallOutputItem,
-    CompactionItem,
     ReasoningItem,
     MCPListToolsItem,
     MCPApprovalRequestItem,