chore: fix proper code format with ./mvnw fmt:format

Author: glaforge

core/src/main/java/com/google/adk/agents/CallbackContext.java

@@ -22,7 +22,6 @@
 import com.google.genai.types.Part;
 import io.reactivex.rxjava3.core.Maybe;
 import io.reactivex.rxjava3.core.Single;
-
 import java.util.List;
 import java.util.Optional;