fix(ios): Guard replay postInit behind runtime session replay check

When iOS deployment target >= 16.0, SENTRY_TARGET_REPLAY_SUPPORTED is
true at compile time, but postInit was called unconditionally even when
session replay was not configured. This caused a SIGABRT crash on launch
in release builds with New Architecture (TurboModules) because the
breadcrumb converter installed by postInit referenced a nil replay
instance.

Fixes #5679

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

Author: antonis

packages/core/ios/RNSentryStart.m

@@ -29,7 +29,9 @@ + (void)startWithOptions:(SentryOptions *)options NS_SWIFT_NAME(start(options:))
     [SentrySDK startWithOptions:options];
 
 #if SENTRY_TARGET_REPLAY_SUPPORTED
-    [RNSentryReplay postInit];
+    if (options.sessionReplay.sessionSampleRate > 0 || options.sessionReplay.onErrorSampleRate > 0) {
+        [RNSentryReplay postInit];
+    }
 #endif
 
     [self postDidBecomeActiveNotification];