refactor: 이전 편지 조회 시 매칭 id 넣어주기 (WR9-125)

Author: Kojinyoung7220

src/main/java/io/crops/warmletter/domain/letter/service/LetterService.java

@@ -115,7 +115,7 @@ public List<LetterResponse> getPreviousLetters(Long letterId) {
 
         if(parentLetterId == null){
             String zipCode = memberRepository.findById(letter.getWriterId()).orElseThrow(MemberNotFoundException::new).getZipCode();
-            LetterResponse response = LetterResponse.fromEntityForPreviousLetters(letter, zipCode, null);
+            LetterResponse response = LetterResponse.fromEntityForPreviousLetters(letter, zipCode, letter.getMatchingId());
             return List.of(response);
 
         }else{