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_test.go

@@ -676,8 +676,8 @@ func TestConstants_值正确(t *testing.T) {
 	if RedirectURI != "http://localhost:8085/callback" {
 		t.Errorf("RedirectURI 不匹配: got %s", RedirectURI)
 	}
-	if UserAgent != "antigravity/1.15.8 windows/amd64" {
-		t.Errorf("UserAgent 不匹配: got %s", UserAgent)
+	if GetUserAgent() != "antigravity/1.84.2 windows/amd64" {
+		t.Errorf("UserAgent 不匹配: got %s", GetUserAgent())
 	}
 	if SessionTTL != 30*time.Minute {
 		t.Errorf("SessionTTL 不匹配: got %v", SessionTTL)