test: FakeMemberLookupService에 findById 메서드 구현 추가

Author: SiwonHae

src/test/java/org/fontory/fontorybe/unit/mock/FakeMemberLookupService.java

@@ -6,6 +6,7 @@
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Optional;
 
 public class FakeMemberLookupService implements MemberLookupService {
     private final Map<Long, Member> members = new HashMap<>();
@@ -25,6 +26,11 @@ public boolean existsByNickname(String nickname) {
                 .anyMatch(member -> member.getNickname().equals(nickname));
     }
 
+    @Override
+    public Optional<Member> findById(Long id) {
+        return Optional.ofNullable(members.get(id));
+    }
+
     // Test helper methods
     public void addMember(Member member) {
         members.put(member.getId(), member);