[client] Stop the gate test from dialing the real management server

TestLogin_RestatingTheStoredConfigPassesTheGate asserts that the gate lets a
no-op login through, and the handler then went on to do the login for real:
isLoginRequired builds an auth client when isLoginRequiredFn is unset, so the
test dialed the profile's management URL — api.netbird.io:443. It took 1.05s
locally and would hang on a runner with no egress, for a fact about the gate
that needs no network at all.

Stubbed like the login_outcome tests do. The test now runs in 0.00s.

Reported by cubic-dev-ai on PR #7398.

client/server/update_settings_gate_test.go

@@ -295,6 +295,11 @@ func TestLogin_RestatingTheStoredConfigPassesTheGate(t *testing.T) {
 	s.updateSettingsDisabled = true
 	s.rootCtx = internal.CtxInitState(context.Background())
 
+	// Stand in for the management round trip the handler makes once the gate
+	// lets it through, so this test exercises the gate and not the network:
+	// without it the profile's management URL is dialed for real.
+	s.isLoginRequiredFn = func(context.Context) (bool, error) { return false, nil }
+
 	_, err := s.Login(userCtx(), &proto.LoginRequest{
 		Username:      &username,
 		ManagementUrl: storedManagementURL,