[FSSDK-12503] fix: guard detachFromEngine from multi-engine channel teardown

[muzahidul-opti]

Summary

• Android: onDetachedFromEngine now checks if the detaching engine's BinaryMessenger matches the one that created the channel — skips cleanup if it doesn't match
• iOS: detachFromEngine now compares registrar.messenger() against the stored attachedMessenger — skips cleanup if they differ
• Fixes the case where a secondary Flutter engine (e.g. Firebase background messaging) detaching would destroy the primary engine's MethodChannel, breaking all SDK calls and notification callbacks

Context

The onAttachedToEngine/register fix from #103 correctly guards against channel overwrite on attach (if channel != null { return }). But onDetachedFromEngine/detachFromEngine unconditionally nils the channel — so when the secondary engine detaches, it destroys the primary engine's channel.

Test plan

• Run multi-engine notification listener integration test on Android emulator
• Run multi-engine notification listener integration test on iOS simulator
• Verify decision listener fires after second engine is created
• Verify track listener fires after second engine is created
• Verify decision listener works after second engine is destroyed (new test case)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Guards native plugin teardown so a secondary Flutter engine (e.g., background messaging) detaching doesn’t destroy the primary engine’s MethodChannel and break SDK calls/callbacks.

Changes:

• iOS: Track the messenger used to create the MethodChannel and skip detachFromEngine cleanup when a different engine is detaching.
• Android: Track the BinaryMessenger that created the MethodChannel and skip onDetachedFromEngine cleanup when the detaching engine uses a different messenger.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ios/Classes/SwiftOptimizelyFlutterSdkPlugin.swift	Stores the attached messenger and gates channel cleanup in detachFromEngine based on messenger identity.
android/src/main/java/com/optimizely/optimizely_flutter_sdk/OptimizelyFlutterSdkPlugin.java	Stores the attached BinaryMessenger and gates channel cleanup in onDetachedFromEngine based on messenger identity.

Comments suppressed due to low confidence (2)

ios/Classes/SwiftOptimizelyFlutterSdkPlugin.swift:56

• In register(with:), you now store let messenger = registrar.messenger(), but the logger channel setup still calls registrar.messenger() again. Reuse the messenger variable (including for makeBackgroundTaskQueue) so both channels are guaranteed to use the same messenger instance and to avoid redundant lookups.

        let messenger = registrar.messenger()
        attachedMessenger = messenger
        channel = FlutterMethodChannel(name: "optimizely_flutter_sdk", binaryMessenger: messenger)
        let instance = SwiftOptimizelyFlutterSdkPlugin()
        registrar.addMethodCallDelegate(instance, channel: channel)

        // Separate logger channel for outgoing log calls
        let taskQueue = registrar.messenger().makeBackgroundTaskQueue?()
        let loggerChannel = FlutterMethodChannel(name: OptimizelyFlutterLogger.LOGGER_CHANNEL, 
                                                binaryMessenger: registrar.messenger(), 
                                                codec: FlutterStandardMethodCodec.sharedInstance(),
                                                taskQueue: taskQueue)

android/src/main/java/com/optimizely/optimizely_flutter_sdk/OptimizelyFlutterSdkPlugin.java:226

• onAttachedToEngine stores the engine messenger in attachedMessenger, but the logger channel is still created with a fresh binding.getBinaryMessenger() call. Prefer using attachedMessenger for the logger channel as well to ensure both channels are tied to the same messenger instance and to avoid unnecessary duplicate lookups.

    attachedMessenger = binding.getBinaryMessenger();
    channel = new MethodChannel(attachedMessenger, "optimizely_flutter_sdk");
    channel.setMethodCallHandler(this);
    context = binding.getApplicationContext();

    MethodChannel loggerChannel = new MethodChannel(binding.getBinaryMessenger(), FlutterLogbackAppender.CHANNEL_NAME);
    FlutterLogbackAppender.setChannel(loggerChannel);

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