fix: 既梦API直接使用图片URL，不再转Base64

- 删除urlToBase64函数
- 直接传递图片URL给既梦API（API原生支持）
- 只有前端上传的Base64图片才需要先上传S3获取URL
- 大幅减少请求体大小和处理时间

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/ai/providers/volcano-vision.ts

@@ -191,37 +191,6 @@ function formatImageResponse(images: Array<{ thumbnail: string; original: string
     .join("\n\n");
 }
 
-/**
- * 下载图片并转换为 Base64
- */
-async function urlToBase64(url: string): Promise<string> {
-  try {
-    console.log("[Volcano Vision] Converting image to Base64:", url.substring(0, 100) + "...");
-    const response = await fetch(url);
-    if (!response.ok) {
-      throw new Error(`Failed to fetch image: ${response.status} ${response.statusText}`);
-    }
-    
-    const arrayBuffer = await response.arrayBuffer();
-    const buffer = Buffer.from(arrayBuffer);
-    const base64 = buffer.toString('base64');
-    
-    // 从 URL 推断图片格式
-    let format = 'jpeg'; // 默认
-    if (url.includes('.png')) format = 'png';
-    else if (url.includes('.webp')) format = 'webp';
-    else if (url.includes('.gif')) format = 'gif';
-    else if (url.includes('.jpg') || url.includes('.jpeg')) format = 'jpeg';
-    
-    const result = `data:image/${format};base64,${base64}`;
-    console.log("[Volcano Vision] ✅ Converted to Base64 (format:", format, ", size:", (base64.length / 1024).toFixed(2), "KB)");
-    return result;
-  } catch (error) {
-    console.error("[Volcano Vision] ❌ Failed to convert image to Base64:", error);
-    throw error;
-  }
-}
-
 /**
  * 将消息转换为火山引擎 API 格式
  */
@@ -274,16 +243,19 @@ async function convertMessagesToVolcanoFormat(messages: any[]): Promise<{ prompt
   console.log("[Volcano Vision] Extracted prompt:", prompt.substring(0, 100));
   console.log("[Volcano Vision] Extracted images count:", images.length);
   
-  // 将所有图片 URL 转换为 Base64
+  // 🆕 直接使用图片URL，不再转换Base64（既梦API支持URL格式）
-  const base64Images: string[] = [];
+  // 只有当图片是Base64格式时才保持原样
+  const processedImages: string[] = [];
   for (const imageUrl of images) {
-    // 如果已经是 Base64 格式，直接使用
     if (imageUrl.startsWith('data:image')) {
-      base64Images.push(imageUrl);
+      // Base64格式：需要先上传到S3获取URL
+      console.log("[Volcano Vision] Found Base64 image, uploading to S3...");
+      const uploaded = await uploadImageToS3(imageUrl);
+      processedImages.push(uploaded.original);
     } else {
-      // 否则下载并转换
+      // URL格式：直接使用
-      const base64Image = await urlToBase64(imageUrl);
+      processedImages.push(imageUrl);
-      base64Images.push(base64Image);
+      console.log("[Volcano Vision] Using image URL directly:", imageUrl.substring(0, 80) + "...");
     }
   }
   
@@ -295,10 +267,10 @@ async function convertMessagesToVolcanoFormat(messages: any[]): Promise<{ prompt
   // 构建请求体
   const result: { prompt: string; image?: string | string[] } = { prompt: enhancedPrompt };
   
-  if (base64Images.length === 1) {
+  if (processedImages.length === 1) {
-    result.image = base64Images[0];
+    result.image = processedImages[0];
-  } else if (base64Images.length > 1) {
+  } else if (processedImages.length > 1) {
-    result.image = base64Images;  // 多图直接传数组
+    result.image = processedImages;  // 多图直接传数组
   }
   
   return result;
@@ -319,7 +291,7 @@ async function sendVolcanoRequest(
   console.log("[Volcano Vision] Sending non-stream request to:", url);
   console.log("[Volcano Vision] Using model:", modelId);
   
-  // 转换消息格式（含 Base64 转换）
+  // 转换消息格式（直接使用URL）
   let { prompt, image } = await convertMessagesToVolcanoFormat(messages);
   
   // 根据 maxImages 参数优化提示词
@@ -452,7 +424,7 @@ async function* sendVolcanoStreamRequest(
   console.log("[Volcano Vision] Sending stream request to:", url);
   console.log("[Volcano Vision] Using model:", modelId);
   
-  // 转换消息格式（含 Base64 转换）
+  // 转换消息格式（直接使用URL）
   const { prompt, image } = await convertMessagesToVolcanoFormat(messages);
   
   console.log("[Volcano Vision] Prompt:", prompt.substring(0, 100) + "...");