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/service/model_rate_limit_test.go

@@ -107,12 +107,12 @@ func TestIsModelRateLimited(t *testing.T) {
 			expected:       true,
 		},
 		{
-			name: "antigravity platform - gemini-3-pro-preview mapped to gemini-3-pro-high",
+			name: "antigravity platform - gemini-3-pro-preview mapped to gemini-3.1-pro-high",
 			account: &Account{
 				Platform: PlatformAntigravity,
 				Extra: map[string]any{
 					modelRateLimitsKey: map[string]any{
-						"gemini-3-pro-high": map[string]any{
+						"gemini-3.1-pro-high": map[string]any{
 							"rate_limit_reset_at": future,
 						},
 					},