autoresearch iter 3: share upload-record error mapping via filesrv helper

ServeFileByID and DownloadFile duplicated the lookup failure mapping and
each used an unchecked *strconv.NumError assertion that cannot match a
wrapped error. One helper now classifies 404 vs 400 via errors.As; each
endpoint keeps its own fallback for unclassified failures. ErrInvalidUploadID
became unused once both sites report ErrInvalidFileID for a malformed ID.

backend/plugins/domain/upload/filesrv/file_server.go

@@ -76,12 +76,7 @@ const (
 func ServeFileByID(c *gin.Context) {
 	upload, err := GetUploadRecordByID(c)
 	if err != nil {
-		if repository.IsRecordNotFound(err) {
-			response.AbortNotFound(c, shared.ErrFileRecordNotFound)
-			return
-		}
-		if _, ok := err.(*strconv.NumError); ok {
-			response.AbortBadRequest(c, shared.ErrInvalidUploadID)
+		if AbortUploadRecordError(c, err) {
 			return
 		}
 		response.AbortInternal(c, shared.ErrInternalServerError)
@@ -96,6 +91,23 @@ func ServeFileByID(c *gin.Context) {
 	ServeUpload(c, upload)
 }
 
+// AbortUploadRecordError maps an upload record lookup failure to its HTTP
+// response and reports whether it handled the error. A missing record is 404
+// and a malformed path ID is 400; anything else is left to the caller.
+func AbortUploadRecordError(c *gin.Context, err error) bool {
+	var numErr *strconv.NumError
+	switch {
+	case repository.IsRecordNotFound(err):
+		response.AbortNotFound(c, shared.ErrFileRecordNotFound)
+		return true
+	case errors.As(err, &numErr):
+		response.AbortBadRequest(c, shared.ErrInvalidFileID)
+		return true
+	default:
+		return false
+	}
+}
+
 // GetUploadRecordByID 从请求路径参数中解析文件 ID 并从数据库中检索处于 Pending 或 Used 状态的上传记录。
 func GetUploadRecordByID(c *gin.Context) (*models.Upload, error) {
 	c.Header("X-Content-Type-Options", "nosniff")

backend/plugins/domain/upload/handler/routers.go

@@ -169,12 +169,7 @@ func UploadFile(c *gin.Context) {
 func DownloadFile(c *gin.Context) {
 	upload, err := filesrv.GetUploadRecordByID(c)
 	if err != nil {
-		if isRecordNotFound(err) {
-			response.AbortNotFound(c, shared.ErrFileRecordNotFound)
-			return
-		}
-		if _, ok := err.(*strconv.NumError); ok {
-			response.AbortBadRequest(c, shared.ErrInvalidFileID)
+		if filesrv.AbortUploadRecordError(c, err) {
 			return
 		}
 		response.AbortBadRequest(c, shared.ErrQueryUploadRecordFailed)

backend/plugins/domain/upload/shared/errs.go

@@ -43,7 +43,6 @@ const (
 	ErrFileRecordNotFound              = "文件记录未找到"
 	ErrFileNotFound                    = "文件未找到"
 	ErrOperationForbidden              = "无权操作"
-	ErrInvalidUploadID                 = "无效的上传ID"
 	ErrInvalidAccessModeParam          = "无效的 access_mode 参数"
 	ErrInternalServerError             = "服务器内部错误"
 )