Fix C++ compatibility workflow: build fat jar with libgdx dependencies

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

.github/workflows/spine-cpp-compatibility.yml

@@ -35,9 +35,9 @@ jobs:
         key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
         restore-keys: ${{ runner.os }}-gradle
 
-    - name: Build spine-libgdx first
+    - name: Build spine-libgdx and test fat jar
       working-directory: spine-libgdx
-      run: ./gradlew build
+      run: ./gradlew build :spine-libgdx-tests:fatJar
 
     - name: Run C++ vs Java compatibility test
       working-directory: tests