fix analysis pdf service cache property declaration

Declare knowledgePointMetaCache explicitly to avoid dynamic property warnings during v3 analysis PDF generation.

Made-with: Cursor

app/Services/ExamPdfExportService.php

@@ -37,6 +37,7 @@ class ExamPdfExportService
      */
     private array $pdfImageDimensionCache = [];
     private ?bool $hasPdfImageMetricsTable = null;
+    private ?array $knowledgePointMetaCache = null;
 
     public function __construct(
         private readonly LearningAnalyticsService $learningAnalyticsService,