Fix NoClassDefFoundError for DefaultJsonSerializer$LazyHolder in ProGuard/R8 release builds

[Copilot]

Summary

Fixes NoClassDefFoundError: com.optimizely.ab.event.internal.serializer.DefaultJsonSerializer$LazyHolder that prevents experiment impression events and conversion/purchase events from being dispatched in minified release builds (Android SDK 5.1.1+).

Root Cause

LogEvent.getBody() (called from EventWorker.getData()) invokes DefaultJsonSerializer.getInstance(), which accesses DefaultJsonSerializer$LazyHolder.INSTANCE using the initialization-on-demand holder (lazy initialization) pattern:

private static class LazyHolder {
    private static final Serializer INSTANCE = create();
}

proguard-rules.txt already keeps com.optimizely.ab.event.internal.payload.** (the event payload model classes), but had no keep rule for com.optimizely.ab.event.internal.serializer.**. R8/ProGuard therefore strips DefaultJsonSerializer and its $LazyHolder inner class during the release build, causing a NoClassDefFoundError at runtime the first time an event is dispatched.

Because the exception occurs before the HTTP request is made, no events reach the /v1/events endpoint — explaining why impression and conversion events silently fail while a custom EventHandler that manually serializes with Gson works fine.

Changes

Added three ProGuard/R8 keep rules to proguard-rules.txt (which is already wired as consumerProguardFiles in the android-sdk module):

Rule	Why
-keep class ...DefaultJsonSerializer { *; }	Entry point called by LogEvent.getBody()
-keep class ...DefaultJsonSerializer$LazyHolder { *; }	Inner class stripped by R8 — directly causes the NoClassDefFoundError
-keep class ...GsonSerializer { *; }	The implementation DefaultJsonSerializer.create() selects at runtime on Android (Gson is bundled; Jackson is not)

Testing

This is a ProGuard-only configuration change. The fix can be verified by building a minified release APK (minifyEnabled true) and confirming:

1. No NoClassDefFoundError in logcat when decide() or track() is called.
2. Impression and conversion events appear in the Optimizely dashboard without a custom EventHandler.