fix(test): mock gateway health check in status tests

Two gateway tests were flaky due to race conditions:
- TestGatewayStatusReturnsRestartingDuringRestartGap
- TestGatewayRestartReturnsErrorStatusWhenReplacementFailsToStart

The handleGatewayStatus function calls getGatewayHealth which can
override the test's expected status. By mocking gatewayHealthGet
to return an error, the tests now reliably verify the expected
status values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/backend/api/gateway_test.go

@@ -596,6 +596,11 @@ func TestGatewayStatusReturnsErrorAfterStartupWindowExpires(t *testing.T) {
 func TestGatewayStatusReturnsRestartingDuringRestartGap(t *testing.T) {
 	resetGatewayTestState(t)
 
+	// Mock health check to return error, so it won't override our "restarting" status
+	gatewayHealthGet = func(url string, timeout time.Duration) (*http.Response, error) {
+		return nil, errors.New("mock health check error")
+	}
+
 	configPath := filepath.Join(t.TempDir(), "config.json")
 	h := NewHandler(configPath)
 	mux := http.NewServeMux()
@@ -738,6 +743,11 @@ func TestGatewayRestartKeepsOldProcessWhenItDoesNotExitInTime(t *testing.T) {
 func TestGatewayRestartReturnsErrorStatusWhenReplacementFailsToStart(t *testing.T) {
 	resetGatewayTestState(t)
 
+	// Mock health check to return error, so it won't override our "error" status
+	gatewayHealthGet = func(url string, timeout time.Duration) (*http.Response, error) {
+		return nil, errors.New("mock health check error")
+	}
+
 	configPath := filepath.Join(t.TempDir(), "config.json")
 	cfg := config.DefaultConfig()
 	cfg.Agents.Defaults.ModelName = cfg.ModelList[0].ModelName