fix: use doNothing for void Panache persist() in verification test example

Panache persist() returns void, so when().thenReturn() won't compile.
Replaced with doNothing().when().persist() which is the correct
Mockito pattern for void methods.

docs/tr/skills/quarkus-verification/SKILL.md

@@ -83,11 +83,9 @@ class UserServiceTest {
   @Test
   void createUser_validInput_returnsUser() {
     var dto = new CreateUserDto("Alice", "alice@example.com");
-    var expected = new User();
-    expected.id = 1L;
-    expected.name = dto.name();
-    
-    when(userRepository.persist(any(User.class))).thenReturn(expected);
+
+    // Panache persist() void döndürür — doNothing + verify kullanın
+    doNothing().when(userRepository).persist(any(User.class));
 
     User result = userService.create(dto);