refactor: simplify optional container path validation

internal/backendexecutor/executor.go

@@ -355,15 +355,11 @@ func validateRequest(request Request) error {
 	if request.PortEnvironmentKey == "" || strings.Contains(request.PortEnvironmentKey, "=") || strings.TrimSpace(request.PortEnvironmentKey) != request.PortEnvironmentKey {
 		return errors.New("exact port environment key is required")
 	}
-	if request.ConfigSource != "" || request.ConfigTarget != "" {
-		if !filepath.IsAbs(request.ConfigSource) || !filepath.IsAbs(request.ConfigTarget) {
-			return errors.New("backend configuration source and target must be absolute paths")
-		}
-	}
-	if request.TmpSource != "" || request.TmpTarget != "" {
-		if !filepath.IsAbs(request.TmpSource) || !filepath.IsAbs(request.TmpTarget) {
-			return errors.New("backend temporary source and target must be absolute paths")
+	if err := validateOptionalPathPair(request.ConfigSource, request.ConfigTarget, "backend configuration source and target"); err != nil {
+		return err
 	}
+	if err := validateOptionalPathPair(request.TmpSource, request.TmpTarget, "backend temporary source and target"); err != nil {
+		return err
 	}
 	if request.ConfigSource != "" {
 		if request.ConfigEnvironmentKey == "" || strings.Contains(request.ConfigEnvironmentKey, "=") || strings.TrimSpace(request.ConfigEnvironmentKey) != request.ConfigEnvironmentKey {
@@ -432,6 +428,17 @@ func directDirectory(path, description string) error {
 	return nil
 }
 
+// validateOptionalPathPair accepts two empty paths or two clean absolute paths.
+func validateOptionalPathPair(source string, target string, description string) error {
+	if source == "" && target == "" {
+		return nil
+	}
+	if !filepath.IsAbs(source) || !filepath.IsAbs(target) {
+		return fmt.Errorf("%s must be absolute paths", description)
+	}
+	return nil
+}
+
 // containerSpec 根据请求构造后端容器的完整规格，包括名称、镜像引用、平台、环境变量、
 // 宿主机网络模式、重启策略、绑定挂载、用户与停止超时。
 func containerSpec(request Request) containerengine.ContainerSpec {