fix: remove type annotation from final_usage to match upstream style

Author: pgrayy

src/strands/models/openai_responses.py

@@ -218,7 +218,7 @@ async def stream(
                 yield self._format_chunk({"chunk_type": "message_start"})
 
                 tool_calls: dict[str, _ToolCallInfo] = {}
-                final_usage: Any = None
+                final_usage = None
                 data_type: str | None = None
                 stop_reason: str | None = None