Identity: Adding user to role gives an Invalid Username error (UserManager.cs).

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

Its possible to create users with Turkish characters like ı or even with whitespaces. But when assigning a role through the UserManager returns an identityerror with Invalid Username.

### Expected Behavior

Example:
create user with username: Yazıcı .
> user created.

assign role to user with UserManager

> Role assigned to User.

### Steps To Reproduce

Example:
create user with username: Yazıcı .
> user created.

assign role to user with UserManager

> returns identity error with the following info:
Code: InvalidUserName.
Description: Username 'Yazıcı ' is invalid, can only contain letters or digits.

### Exceptions (if any)

no exceptions

### .NET Version

6

### Anything else?

_No response_

Start with UserManager.cs and reproduce the reported flow using the username “Yazıcı ”: create the user, then assign a role through UserManager. Compare the validation performed in each operation; done means role assignment succeeds consistently for usernames accepted during creation, with regression coverage added where the existing Identity tests belong.

Written by the indexing model from the issue text.