refactor(knowledge-item): 优化知识项预览文件路径处理逻辑

- 将文件路径验证逻辑从方法开始位置移动到实际使用位置
- 修复了预览文件名获取方式，直接从相对路径解析文件名
- 确保文件存在性检查只在需要时执行
- 提高了代码可读性和执行效率

backend/services/data-management-service/src/main/java/com/datamate/datamanagement/application/KnowledgeItemApplicationService.java

@@ -366,12 +366,9 @@ public class KnowledgeItemApplicationService {
 
         String relativePath = knowledgeItem.getContent();
         BusinessAssert.isTrue(StringUtils.isNotBlank(relativePath), CommonErrorCode.PARAM_ERROR);
-        Path filePath = resolveKnowledgeItemStoragePath(relativePath);
-        BusinessAssert.isTrue(Files.exists(filePath) && Files.isRegularFile(filePath), CommonErrorCode.PARAM_ERROR);
-
         String previewName = StringUtils.isNotBlank(knowledgeItem.getSourceFileId())
             ? knowledgeItem.getSourceFileId()
-            : filePath.getFileName().toString();
+            : Paths.get(relativePath).getFileName().toString();
 
         if (knowledgeItemPreviewService.isOfficeDocument(previewName)) {
             KnowledgeItemPreviewService.PreviewFile previewFile = knowledgeItemPreviewService.resolveReadyPreviewFile(setId, knowledgeItem);
@@ -393,6 +390,9 @@ public class KnowledgeItemApplicationService {
             return;
         }
 
+        Path filePath = resolveKnowledgeItemStoragePath(relativePath);
+        BusinessAssert.isTrue(Files.exists(filePath) && Files.isRegularFile(filePath), CommonErrorCode.PARAM_ERROR);
+
         String contentType = null;
         try {
             contentType = Files.probeContentType(filePath);