fixed typecheck

Author: wrisa

instrumentation-genai/opentelemetry-instrumentation-langchain/src/opentelemetry/instrumentation/langchain/span_manager.py

@@ -97,6 +97,6 @@ def end_span(self, run_id: UUID) -> None:
             # Always end the span as OpenTelemetry spans don't expose end_time directly
         state.span.end()
 
-    def get_span(self, run_id: UUID) -> Span:
+    def get_span(self, run_id: UUID) -> Optional[Span]:
         state = self.spans.get(run_id)
         return state.span if state else None