fix: add iOS engine detach cleanup and enforce private channel access

Add detachFromEngine(for:) to iOS plugin so channel is cleaned up when
the engine detaches, matching Android's onDetachedFromEngine behavior.
Make FlutterLogbackAppender.channel private to enforce use of
setChannel/clearChannel accessors.

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

Author: muzahidul-opti

android/src/main/java/com/optimizely/optimizely_flutter_sdk/FlutterLogbackAppender.java

@@ -14,7 +14,7 @@
 public class FlutterLogbackAppender extends AppenderBase<ILoggingEvent> {
 
     public static final String CHANNEL_NAME = "optimizely_flutter_sdk_logger";
-    public static MethodChannel channel;
+    private static MethodChannel channel;
     private static final Handler mainThreadHandler = new Handler(Looper.getMainLooper());
 
     public static void setChannel(MethodChannel newChannel) {

ios/Classes/SwiftOptimizelyFlutterSdkPlugin.swift

@@ -53,7 +53,13 @@ public class SwiftOptimizelyFlutterSdkPlugin: NSObject, FlutterPlugin {
                                                 taskQueue: taskQueue)
         OptimizelyFlutterLogger.setChannel(loggerChannel)
     }
-    
+
+    public func detachFromEngine(for registrar: FlutterPluginRegistrar) {
+        Self.channel?.setMethodCallHandler(nil)
+        Self.channel = nil
+        OptimizelyFlutterLogger.clearChannel()
+    }
+
     /// Part of FlutterPlugin protocol to handle communication with flutter sdk.
     /// All method handlers receive a main-thread-safe result callback so that
     /// any handler calling result() from a background thread (e.g. async SDK