Refactor constants and add private constructor in JsonConf.

Changed API_EMAIL and API_KEY to static final for proper constant usage. Added a private constructor in JsonConf to prevent instantiation, ensuring adherence to utility class design principles.

src/main/java/codes/thischwa/cf/JsonConf.java

@@ -12,6 +12,9 @@ import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
  */
 class JsonConf {
 
+  private JsonConf() {
+  }
+
   static ObjectMapper initObjectMapper() {
     ObjectMapper mapper = new ObjectMapper();
     mapper.registerModule(new JavaTimeModule());

src/test/java/codes/thischwa/cf/CfClientTest.java

@@ -1,14 +1,17 @@
 package codes.thischwa.cf;
 
-import static org.junit.jupiter.api.Assertions.*;
-
 import codes.thischwa.cf.model.RecordEntity;
 import codes.thischwa.cf.model.RecordType;
 import codes.thischwa.cf.model.ZoneEntity;
-
 import java.time.LocalDate;
 import java.util.List;
 import lombok.extern.slf4j.Slf4j;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
 import org.junit.jupiter.api.Test;
 
 @Slf4j
@@ -18,8 +21,8 @@ public class CfClientTest {
   private static final String SLD_STR = "devsld";
   private static final int TTL = 60;
 
-  private final String API_EMAIL = System.getenv("API_EMAIL");
+  private static final String API_EMAIL = System.getenv("API_EMAIL");
-  private final String API_KEY = System.getenv("API_KEY");
+  private static final String API_KEY = System.getenv("API_KEY");
 
   private final CfDnsClient client = new CfDnsClient(API_EMAIL, API_KEY);