refactor: extract Serializer's JsonMapper into an explicit adapter class

src/main/java/edu/byu/cs/server/SerializerAdapter.java

@@ -0,0 +1,25 @@
+package edu.byu.cs.server;
+
+import edu.byu.cs.util.Serializer;
+import io.javalin.json.JsonMapper;
+import org.jetbrains.annotations.NotNull;
+
+import java.lang.reflect.Type;
+
+/**
+ * A Serializer class adapter which implements Javalin's JsonMapper interface.
+ * Allows Javalin's built-in serialization to use Serializer.
+ */
+public class SerializerAdapter implements JsonMapper {
+    @NotNull
+    @Override
+    public <T> T fromJsonString(@NotNull String json, @NotNull Type targetType) {
+        return Serializer.deserialize(json, targetType);
+    }
+
+    @NotNull
+    @Override
+    public String toJsonString(@NotNull Object obj, @NotNull Type type) {
+        return Serializer.serialize(obj, type);
+    }
+}

src/main/java/edu/byu/cs/server/Server.java

@@ -3,7 +3,6 @@
 import edu.byu.cs.controller.WebSocketController;
 import edu.byu.cs.controller.exception.*;
 import edu.byu.cs.server.endpointprovider.EndpointProvider;
-import edu.byu.cs.util.Serializer;
 import io.javalin.Javalin;
 import io.javalin.http.ExceptionHandler;
 import io.javalin.http.HandlerType;
@@ -33,7 +32,7 @@ private int setupEndpoints(int port) {
         app = Javalin.create(config -> {
                     config.staticFiles.add("/frontend/dist");
 
-                    config.jsonMapper(Serializer.jsonMapper);
+                    config.jsonMapper(new SerializerAdapter());
 
                     config.router.apiBuilder(() -> {
                         before(provider.beforeAll());

src/main/java/edu/byu/cs/util/Serializer.java

@@ -6,8 +6,6 @@
 import com.google.gson.stream.JsonWriter;
 import edu.byu.cs.canvas.model.CanvasRubricAssessment;
 import edu.byu.cs.canvas.model.CanvasRubricItem;
-import io.javalin.json.JsonMapper;
-import org.jetbrains.annotations.NotNull;
 
 import java.io.IOException;
 import java.io.Reader;
@@ -25,20 +23,6 @@ public class Serializer {
             .registerTypeAdapter(CanvasRubricAssessment.class, new RubricAssessmentAdapter())
             .create();
 
-    public static final JsonMapper jsonMapper = new JsonMapper() {
-        @NotNull
-        @Override
-        public <T> T fromJsonString(@NotNull String json, @NotNull Type targetType) {
-            return deserialize(json, targetType);
-        }
-
-        @NotNull
-        @Override
-        public String toJsonString(@NotNull Object obj, @NotNull Type type) {
-            return serialize(obj, type);
-        }
-    };
-
     public static String serialize(Object obj) {
         try {
             return GSON.toJson(obj);