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/images_test.go

@@ -0,0 +1,102 @@
+package downloader
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestIsImageURL(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected bool
+	}{
+		{"https://example.com/image.jpg", true},
+		{"http://example.com/image.png", true},
+		{"HTTPS://example.com/image.gif", true},
+		{"/local/path/image.jpg", false},
+		{"./relative/path/image.png", false},
+		{"image.jpg", false},
+		{"ftp://example.com/image.jpg", false},
+		{"", false},
+	}
+
+	for _, test := range tests {
+		result := IsImageURL(test.input)
+		if result != test.expected {
+			t.Errorf("IsImageURL(%q) = %v, expected %v", test.input, result, test.expected)
+		}
+	}
+}
+
+func TestNewImageDownloader(t *testing.T) {
+	tempDir := os.TempDir()
+	testPath := filepath.Join(tempDir, "test_downloader")
+	defer os.RemoveAll(testPath)
+
+	downloader := NewImageDownloader(testPath)
+
+	if downloader == nil {
+		t.Fatal("NewImageDownloader returned nil")
+	}
+
+	if downloader.savePath != testPath {
+		t.Errorf("savePath = %q, expected %q", downloader.savePath, testPath)
+	}
+
+	// 验证目录是否创建
+	if _, err := os.Stat(testPath); os.IsNotExist(err) {
+		t.Errorf("save path directory was not created: %s", testPath)
+	}
+}
+
+func TestImageDownloader_isValidImageURL(t *testing.T) {
+	downloader := NewImageDownloader(os.TempDir())
+
+	tests := []struct {
+		url      string
+		expected bool
+	}{
+		{"https://example.com/image.jpg", true},
+		{"http://example.com/image.png", true},
+		{"https://", false},
+		{"http://", false},
+		{"invalid-url", false},
+		{"ftp://example.com/image.jpg", false},
+		{"", false},
+	}
+
+	for _, test := range tests {
+		result := downloader.isValidImageURL(test.url)
+		if result != test.expected {
+			t.Errorf("isValidImageURL(%q) = %v, expected %v", test.url, result, test.expected)
+		}
+	}
+}
+
+func TestImageDownloader_generateFileName(t *testing.T) {
+	downloader := NewImageDownloader(os.TempDir())
+
+	url := "https://example.com/image.jpg"
+	extension := "jpg"
+
+	fileName1 := downloader.generateFileName(url, extension)
+
+	// 文件名应该包含扩展名
+	if filepath.Ext(fileName1) != "."+extension {
+		t.Errorf("fileName should end with .%s, got %s", extension, fileName1)
+	}
+
+	// 文件名应该包含img_前缀
+	if !strings.HasPrefix(filepath.Base(fileName1), "img_") {
+		t.Errorf("fileName should start with img_, got %s", fileName1)
+	}
+
+	// 不同URL应该生成不同的文件名
+	url2 := "https://example.com/different.jpg"
+	fileName2 := downloader.generateFileName(url2, extension)
+	if fileName1 == fileName2 {
+		t.Errorf("different URLs should generate different file names")
+	}
+}