test: fix test message length to meet 10-character minimum

Changed test message from "문의합니다." (5 chars) to
"사업자 세무 관련해서 문의드립니다." (18 chars) to comply
with the new 10-character minimum message length validation.

All tests now pass: 26/26 ✓

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Application.Tests/InquiryServiceTests.cs

@@ -22,7 +22,7 @@ public class InquiryServiceTests
         var repository = new FakeInquiryRepository();
         var service = new InquiryService(repository, new FakeInquiryNotificationService(), new MemoryCache(new MemoryCacheOptions()));
 
-        await service.SubmitAsync("홍길동", "010-1234-5678", "기장", "문의합니다.", "user@example.com");
+        await service.SubmitAsync("홍길동", "010-1234-5678", "기장", "사업자 세무 관련해서 문의드립니다.", "user@example.com");
 
         Assert.Equal("user@example.com", repository.Inquiries.Single().Email);
         Assert.Equal("new", repository.Inquiries.Single().Status);