[Fix] DIRECT 채팅방 재입장 및 메시지 전송 시 ChatPart 복구

src/main/java/com/capstone/pickIt/api/chat/service/ChatMessageCommandServiceImpl.java

@@ -155,6 +155,12 @@ public void sendMessage(Long currentUserId, ChatMessageSendRequestDTO request) {
                     currentUserId
             );
 
+            // DIRECT 채팅 상대방이 나간 상태라면 채팅방 재노출을 위해 복구
+            if (chatRoom.getChatType() == ChatType.DIRECT) {
+                chatPartRepository.findOpponentIncludingDeleted(chatRoom.getId(), currentUserId)
+                        .ifPresent(ChatPart::restore);
+            }
+
             // 채팅 목록(lastMessage, unreadCount) 갱신용 개인 알림 이벤트 발행
             eventPublisher.publishEvent(
                     new ChatUserNotificationEvent(receiverId, notification)

src/main/java/com/capstone/pickIt/api/chat/service/ChatRoomCommandServiceImpl.java

@@ -378,7 +378,12 @@ private DirectChatRoomResponseDTO.CreateOrEnter restoreAndConvert(
                 .findByChatRoomIdAndUserId(chatRoom.getId(), currentUserId)
                 .orElseThrow(() -> new ChatException(ChatErrorCode.CHAT_PART_NOT_FOUND));
 
+        ChatPart targetChatPart = chatPartRepository
+                .findByChatRoomIdAndUserId(chatRoom.getId(), targetUser.getId())
+                .orElseThrow(() -> new ChatException(ChatErrorCode.CHAT_PART_NOT_FOUND));
+
         currentChatPart.restore();
+        targetChatPart.restore();
 
         return ChatRoomConverter.toCreateOrEnterResponse(chatRoom, targetUser, isNew);
     }

src/main/java/com/capstone/pickIt/api/chat/service/ChatRoomQueryServiceImpl.java

@@ -436,7 +436,7 @@ private ChatRoomResponseDTO.Opponent getOpponentOrNull(
         }
 
         ChatPart opponentChatPart = chatPartRepository
-                .findOpponent(chatRoomId, currentUserId)
+                .findOpponentIncludingDeleted(chatRoomId, currentUserId)
                 .orElseThrow(() -> new ChatException(ChatErrorCode.CHAT_PART_NOT_FOUND));
 
         return ChatMessageConverter.toOpponent(opponentChatPart);

src/main/java/com/capstone/pickIt/domain/chat/repository/ChatPartRepository.java

@@ -35,7 +35,7 @@ Optional<ChatPart> findActiveOpponent(
             WHERE cp.chatRoom.id = :chatRoomId
               AND cp.user.id <> :currentUserId
             """)
-    Optional<ChatPart> findOpponent(
+    Optional<ChatPart> findOpponentIncludingDeleted(
             Long chatRoomId,
             Long currentUserId
     );