fix(core): Guard against nil SPI result on iOS and leaked listener on Android

- iOS: Add nil check on getFramesDelaySPI result before accessing
  delayDuration (messaging nil returns 0 in ObjC, causing false
  frames.delay: 0)
- Android: Call stop() before start() in RNSentryFrameDelayCollector
  to prevent leaked listeners on repeated initialization (e.g. JS
  bundle reload)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antonis

packages/core/android/src/main/java/io/sentry/react/RNSentryFrameDelayCollector.java

@@ -31,6 +31,7 @@ public boolean start(@Nullable SentryFrameMetricsCollector frameMetricsCollector
     if (frameMetricsCollector == null) {
       return false;
     }
+    stop();
     this.collector = frameMetricsCollector;
     this.listenerId = frameMetricsCollector.startCollection(this);
     return this.listenerId != null;

packages/core/ios/RNSentry.mm

@@ -572,7 +572,7 @@ - (void)handleShakeDetected
     SentryFramesDelayResultSPI *result = [framesTracker getFramesDelaySPI:startSystemTime
                                                        endSystemTimestamp:endSystemTime];
 
-    if (result.delayDuration >= 0) {
+    if (result != nil && result.delayDuration >= 0) {
         resolve(@(result.delayDuration));
     } else {
         resolve(nil);