Relax username length requirement and remove nickname

src/test/java/com/openisle/service/UsernameValidatorTest.java

@@ -0,0 +1,22 @@
+package com.openisle.service;
+
+import com.openisle.exception.FieldException;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class UsernameValidatorTest {
+
+    @Test
+    void rejectsEmptyUsername() {
+        UsernameValidator validator = new UsernameValidator();
+        assertThrows(FieldException.class, () -> validator.validate(""));
+        assertThrows(FieldException.class, () -> validator.validate(null));
+    }
+
+    @Test
+    void allowsShortUsername() {
+        UsernameValidator validator = new UsernameValidator();
+        assertDoesNotThrow(() -> validator.validate("a"));
+    }
+}