fix(samples): Add Retrofit R8 full-mode keep rules to Android sample (#5727)

The Android sample's release build uses R8 full mode, which strips generic signatures from classes that aren't kept. This broke Retrofit call-adapter creation for GithubAPI, crashing SecondActivity in onResume with 'Unable to create call adapter for interface retrofit2.Call'.

Add the standard Retrofit R8 keep rules (Signature attributes, retrofit2.Call/Response, service interfaces) and keep the Repo response model for Gson.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: runningcode

sentry-samples/sentry-samples-android/proguard-rules.pro

@@ -32,3 +32,15 @@
 -dontwarn org.openjsse.javax.net.ssl.SSLParameters
 -dontwarn org.openjsse.javax.net.ssl.SSLSocket
 -dontwarn org.openjsse.net.ssl.OpenJSSE
+
+# Retrofit relies on generic signatures for its service methods. Under R8 full mode these are
+# stripped for classes that aren't kept, which breaks call-adapter creation (SecondActivity's
+# GithubAPI request). Keep the signature attributes and Retrofit's generic types.
+-keepattributes Signature, InnerClasses, EnclosingMethod
+-keep,allowobfuscation,allowshrinking interface retrofit2.Call
+-keep,allowobfuscation,allowshrinking class retrofit2.Response
+-keep,allowobfuscation,allowshrinking class kotlin.coroutines.Continuation
+-if interface * { @retrofit2.http.* <methods>; }
+-keep,allowobfuscation interface <1>
+# Keep the response model so Gson can deserialize it.
+-keep class io.sentry.samples.android.Repo { *; }