Add feedback when trying to add a role a user already has

[brian-smith-tcril]

From #9 (comment)

I did notice the lack of feedback when trying to add a role that a user already has. I see that is mentioned in the PR description as

TODO: Add a toast to inform that the current user has already assigned a role

I'm perfectly fine with it being a follow up issue if it's created on the repo so the TODO doesn't get lost in this PR.

[dcoa]

I made this as part of a different PR:

frontend-app-admin-console/src/authz-module/libraries-manager/components/AssignNewRoleModal/AssignNewRoleTrigger.tsx

Lines 42 to 49 in a0dc22c

	if (currentUserRoles.includes(newRole)) {
	showToast({
	message: intl.formatMessage(messages['libraries.authz.manage.assign.role.existing']),
	type: 'success',
	});
	close();
	setNewRole('');
	return;

Screencast.from.2025-11-03.08-45-34.webm

Do you think we can close this? @brian-smith-tcril