fix: preserve grsai gpt image reference ratio

lib/ai/providers/gemini-image.ts

@@ -70,6 +70,47 @@ async function ensureImageUrl(url: string): Promise<string> {
   return url;
 }
 
+async function inferImageAspectRatioFromUrl(url: string): Promise<string | null> {
+  const normalized = String(url || "").trim();
+  if (!normalized) return null;
+
+  try {
+    let buffer: Buffer | null = null;
+    if (normalized.startsWith("data:")) {
+      const match = normalized.match(/^data:([A-Za-z0-9.+/-]+);base64,(.+)$/);
+      if (!match) return null;
+      buffer = Buffer.from(match[2], "base64");
+    } else if (/^https?:\/\//i.test(normalized)) {
+      const controller = new AbortController();
+      const timeoutId = setTimeout(() => controller.abort(), 20000);
+      try {
+        const response = await fetch(normalized, {
+          signal: controller.signal,
+          headers: {
+            "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36",
+          },
+        });
+        if (!response.ok) return null;
+        buffer = Buffer.from(await response.arrayBuffer());
+      } finally {
+        clearTimeout(timeoutId);
+      }
+    }
+
+    if (!buffer?.length) return null;
+    const sharp = await import("sharp");
+    const metadata = await sharp.default(buffer).metadata();
+    if (!metadata.width || !metadata.height || metadata.height <= 0) {
+      return null;
+    }
+
+    return `${metadata.width}:${metadata.height}`;
+  } catch (error) {
+    console.warn("[Gemini Image] Failed to infer reference aspect ratio:", error);
+    return null;
+  }
+}
+
 /**
  * 将 base64 图片上传到 S3，返回缩略图和原图的 URL
  */
@@ -483,10 +524,19 @@ async function sendGeminiRequest(
   }
   
   if (isGptImageModel) {
-    requestBody.size = resolveGrsaiGptImageSize(imageParams?.aspectRatio, imageParams?.imageSize);
+    const requestedAspectRatio = imageParams?.aspectRatio || "auto";
+    const inferredAspectRatio =
+      requestedAspectRatio === "auto" && processedUrls.length > 0
+        ? await inferImageAspectRatioFromUrl(processedUrls[0])
+        : null;
+    const resolvedAspectRatio = inferredAspectRatio || requestedAspectRatio;
+    requestBody.size = resolveGrsaiGptImageSize(resolvedAspectRatio, imageParams?.imageSize);
     requestBody.shutProgress = false;
+    if (inferredAspectRatio) {
+      console.log(`${apiLogPrefix} Inferred reference aspect ratio: ${inferredAspectRatio}`);
+    }
     console.log(
-      `${apiLogPrefix} Resolved size: aspectRatio=${imageParams?.aspectRatio || "auto"}, imageSize=${imageParams?.imageSize || "2K"} => ${requestBody.size}`
+      `${apiLogPrefix} Resolved size: aspectRatio=${resolvedAspectRatio}, imageSize=${imageParams?.imageSize || "2K"} => ${requestBody.size}`
     );
   } else {
     // 添加 Nano Banana 特定参数