mcp: add image url support (#1)

* refactor: remove redundant validation in service and clean up code

- Remove validatePublishRequest function as gin binding already handles validation
- Remove unused errors import
- Simplify PublishContent method by relying on gin's built-in validation
- Add comprehensive image processing support with URL download capability

* 添加 MCP 说明文档，完善教程

pkg/downloader/processor.go

@@ -0,0 +1,53 @@
+package downloader
+
+import (
+	"fmt"
+
+	"github.com/xpzouying/xiaohongshu-mcp/configs"
+)
+
+// ImageProcessor 图片处理器
+type ImageProcessor struct {
+	downloader *ImageDownloader
+}
+
+// NewImageProcessor 创建图片处理器
+func NewImageProcessor() *ImageProcessor {
+	return &ImageProcessor{
+		downloader: NewImageDownloader(configs.GetImagesPath()),
+	}
+}
+
+// ProcessImages 处理图片列表，返回本地文件路径
+// 支持两种输入格式：
+// 1. URL格式 (http/https开头) - 自动下载到本地
+// 2. 本地文件路径 - 直接使用
+func (p *ImageProcessor) ProcessImages(images []string) ([]string, error) {
+	var localPaths []string
+	var urlsToDownload []string
+
+	// 分离URL和本地路径
+	for _, image := range images {
+		if IsImageURL(image) {
+			urlsToDownload = append(urlsToDownload, image)
+		} else {
+			// 本地路径直接添加
+			localPaths = append(localPaths, image)
+		}
+	}
+
+	// 批量下载URL图片
+	if len(urlsToDownload) > 0 {
+		downloadedPaths, err := p.downloader.DownloadImages(urlsToDownload)
+		if err != nil {
+			return nil, fmt.Errorf("failed to download images: %w", err)
+		}
+		localPaths = append(localPaths, downloadedPaths...)
+	}
+
+	if len(localPaths) == 0 {
+		return nil, fmt.Errorf("no valid images found")
+	}
+
+	return localPaths, nil
+}