fix(feedback): capture onFormClose as local variable in lambda

The onFormClose lambda was reading previousOnFormClose field at dismiss
time. If onActivityResumed or onActivityDestroyed already restored and
nulled the field, the lambda would overwrite onFormClose with null. Now
captured as a local variable at dialog creation time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonis

sentry-android-core/src/main/java/io/sentry/android/core/FeedbackShakeIntegration.java

@@ -145,15 +145,16 @@ private void startShakeDetection(final @NotNull Activity activity) {
                   }
                   try {
                     isDialogShowing = true;
-                    previousOnFormClose = options.getFeedbackOptions().getOnFormClose();
+                    final Runnable captured = options.getFeedbackOptions().getOnFormClose();
+                    previousOnFormClose = captured;
                     options
                         .getFeedbackOptions()
                         .setOnFormClose(
                             () -> {
                               isDialogShowing = false;
-                              options.getFeedbackOptions().setOnFormClose(previousOnFormClose);
-                              if (previousOnFormClose != null) {
-                                previousOnFormClose.run();
+                              options.getFeedbackOptions().setOnFormClose(captured);
+                              if (captured != null) {
+                                captured.run();
                               }
                               previousOnFormClose = null;
                             });