Revert "Re-export extractMessageText for ConversationTraceModal tests"

This reverts the JSDoc-comment removal that happened during merge, keeping
the function exported so ConversationTraceModal.test.ts can import it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/ConversationTraceModal.tsx

@@ -85,7 +85,6 @@ export function ConversationTraceModal({ open, workspaceId: _workspaceId, onClos
     });
   }, [open, nodes]);
 
-  /** Exported for unit testing — see ConversationTraceModal.test.ts */
   const isA2A = (e: ActivityEntry) =>
     e.activity_type === "a2a_receive" || e.activity_type === "a2a_send";