fix: avoid duplicate call to extractReasoningContentFromMessage (#2661)

In convertToSteps(), the extractReasoningContentFromMessage function was
called twice for the same message - once to check if reasoning exists and
again to assign it to the step object. Reuse the already-extracted value
from the local variable instead.

frontend/src/components/workspace/messages/message-group.tsx

@@ -451,7 +451,7 @@ function convertToSteps(messages: Message[]): CoTStep[] {
           id: message.id,
           messageId: message.id,
           type: "reasoning",
-          reasoning: extractReasoningContentFromMessage(message),
+          reasoning,
         };
         steps.push(step);
       }