ASAP-451 편지 수신자 확인 로직 단순화

- `isSameReceiver` 메서드 수정: 람다를 통한 사용자 객체 생성 대신 직접 `User` 객체를 전달받는 방식으로 변경.
- 관련 서비스 로직 리팩토링: 불필요한 람다 호출 제거 및 가독성 향상.
- 예외 처리 방식 유지하며 로직 간결화.

Author: tlarbals824

Application-Module/src/main/kotlin/com/asap/application/letter/service/LetterCommandService.kt

@@ -88,15 +88,15 @@ class LetterCommandService(
         }
 
         val sendLetter = sendLetterManagementPort.getLetterByCodeNotNull(command.letterCode)
-        sendLetter
-            .isSameReceiver {
-                userManagementPort.getUserNotNull(DomainId(command.userId))
-            }.takeIf { it }
-            ?.let {
-                sendLetter.readLetter(DomainId(command.userId))
-                sendLetterManagementPort.save(sendLetter)
-                return VerifyLetterAccessibleUsecase.Response(letterId = sendLetter.id.value)
-            } ?: throw LetterException.InvalidLetterAccessException()
+        val receiver = userManagementPort.getUserNotNull(DomainId(command.userId))
+
+        if (sendLetter.isSameReceiver(receiver)) {
+            sendLetter.readLetter(DomainId(command.userId))
+            sendLetterManagementPort.save(sendLetter)
+            return VerifyLetterAccessibleUsecase.Response(letterId = sendLetter.id.value)
+        }
+
+        throw LetterException.InvalidLetterAccessException()
     }
 
     override fun addVerifiedLetter(command: AddLetterUsecase.Command.VerifyLetter) {

Domain-Module/src/main/kotlin/com/asap/domain/letter/entity/SendLetter.kt

@@ -69,10 +69,7 @@ class SendLetter(
             )
     }
 
-    fun isSameReceiver(receiver: () -> User): Boolean {
-        val receiverUser = receiver()
-        return receiverName == receiverUser.username && (receiverId == null || receiverId == receiverUser.id)
-    }
+    fun isSameReceiver(receiver: User): Boolean = receiverName == receiver.username && (receiverId == null || receiverId == receiver.id)
 
     fun readLetter(receiverId: DomainId) {
         this.receiverId = receiverId