fix: 修复 antigravity UserAgent 重构遗留的编译错误和测试不匹配

- oauth.go: GetUserAgent() 缺少闭合大括号导致语法错误
- client_test.go/oauth_test.go: UserAgent 变量已重构为 GetUserAgent()，更新测试引用
- model_rate_limit_test.go: gemini-3-pro-preview 映射目标已更新为 gemini-3.1-pro-high，同步测试

backend/internal/pkg/antigravity/oauth.go

@@ -64,7 +64,8 @@ func init() {
 // GetUserAgent 返回当前配置的 User-Agent
 func GetUserAgent() string {
 	return fmt.Sprintf("antigravity/%s windows/amd64", defaultUserAgentVersion)
-  
+}
+
 func getClientSecret() (string, error) {
 	if v := strings.TrimSpace(ClientSecret); v != "" {
 		return v, nil