feat(gemini): 为 Gemini 原生平台添加图片计费支持

对齐 Antigravity 平台的图片计费逻辑：
- 添加 extractImageSize() 方法提取图片尺寸
- Forward() 和 ForwardNative() 返回 ImageCount/ImageSize
- 支持分组自定义图片价格和倍率

backend/internal/service/gemini_messages_compat_service.go

@@ -800,6 +800,13 @@ func (s *GeminiMessagesCompatService) Forward(ctx context.Context, c *gin.Contex
 		}
 	}
 
+	// 图片生成计费
+	imageCount := 0
+	imageSize := s.extractImageSize(body)
+	if isImageGenerationModel(originalModel) {
+		imageCount = 1
+	}
+
 	return &ForwardResult{
 		RequestID:    requestID,
 		Usage:        *usage,
@@ -807,6 +814,8 @@ func (s *GeminiMessagesCompatService) Forward(ctx context.Context, c *gin.Contex
 		Stream:       req.Stream,
 		Duration:     time.Since(startTime),
 		FirstTokenMs: firstTokenMs,
+		ImageCount:   imageCount,
+		ImageSize:    imageSize,
 	}, nil
 }
 
@@ -1240,6 +1249,13 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 		usage = &ClaudeUsage{}
 	}
 
+	// 图片生成计费
+	imageCount := 0
+	imageSize := s.extractImageSize(body)
+	if isImageGenerationModel(originalModel) {
+		imageCount = 1
+	}
+
 	return &ForwardResult{
 		RequestID:    requestID,
 		Usage:        *usage,
@@ -1247,6 +1263,8 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 		Stream:       stream,
 		Duration:     time.Since(startTime),
 		FirstTokenMs: firstTokenMs,
+		ImageCount:   imageCount,
+		ImageSize:    imageSize,
 	}, nil
 }
 
@@ -2816,3 +2834,26 @@ func convertClaudeGenerationConfig(req map[string]any) map[string]any {
 	}
 	return out
 }
+
+// extractImageSize 从 Gemini 请求中提取 image_size 参数
+func (s *GeminiMessagesCompatService) extractImageSize(body []byte) string {
+	var req struct {
+		GenerationConfig *struct {
+			ImageConfig *struct {
+				ImageSize string `json:"imageSize"`
+			} `json:"imageConfig"`
+		} `json:"generationConfig"`
+	}
+	if err := json.Unmarshal(body, &req); err != nil {
+		return "2K"
+	}
+
+	if req.GenerationConfig != nil && req.GenerationConfig.ImageConfig != nil {
+		size := strings.ToUpper(strings.TrimSpace(req.GenerationConfig.ImageConfig.ImageSize))
+		if size == "1K" || size == "2K" || size == "4K" {
+			return size
+		}
+	}
+
+	return "2K"
+}