fix(antigravity): add 30s timeout for TestConnection retry loop

Prevents MODEL_CAPACITY_EXHAUSTED 60x1s retries from blocking
test connection for up to 60 seconds. Context timeout ensures
the test connection returns within a reasonable time.

backend/internal/service/antigravity_gateway_service.go

@@ -1043,8 +1043,13 @@ func (s *AntigravityGatewayService) TestConnection(ctx context.Context, account
 
 	// 复用 antigravityRetryLoop：完整的重试 / credits overages / 智能重试
 	prefix := fmt.Sprintf("[antigravity-Test] account=%d(%s)", account.ID, account.Name)
+	// 限制测试连接的总超时：避免 MODEL_CAPACITY_EXHAUSTED 60 次重试导致长时间阻塞
+	const testConnectionTimeout = 30 * time.Second
+	testCtx, cancel := context.WithTimeout(ctx, testConnectionTimeout)
+	defer cancel()
+
 	p := antigravityRetryLoopParams{
-		ctx:            ctx,
+		ctx:            testCtx,
 		prefix:         prefix,
 		account:        account,
 		proxyURL:       proxyURL,