fix(protocol): Grok 生图 - 让 response_format 开关生效 (#692)

backend/internal/app/provider_image.go

@@ -303,7 +303,7 @@ func grokImageRequestBody(input canvasGenerationInput) (grokImageRequest, string
 		Model:          input.Config.Model,
 		Prompt:         withSystemPrompt(input.Config, input.Prompt),
 		N:              1,
-		ResponseFormat: "url",
+		ResponseFormat: grokImageResponseFormat(input.ImageCapability),
 		// Grok 图片协议用 aspect_ratio 表达画布比例；同时发送 size 会被上游按 OpenAI 枚举校验并拒绝。
 		AspectRatio: normalizeGrokImageAspectRatio(input.Config.Size),
 		Resolution:  normalizeGrokImageResolution(input.Config.Quality),
@@ -322,6 +322,13 @@ func grokImageRequestBody(input canvasGenerationInput) (grokImageRequest, string
 	return body, "/images/edits", nil
 }
 
+func grokImageResponseFormat(profile *ImageCapabilityConfig) string {
+	if imageParameterSupported(profile, "response_format") {
+		return "b64_json"
+	}
+	return "url"
+}
+
 // normalizeGrokImageResolution 把画布 quality（1k/2k/high…）映射为 grok2api / xAI 的 resolution。
 func normalizeGrokImageResolution(quality string) string {
 	raw := strings.ToLower(strings.TrimSpace(quality))