Data returned from the API on StagesApi().stages_invitation_invitations_create() fails validation

With `authentik_client` version 2025.10.3 when using `StagesApi().stages_invitation_invitations_create()` the client raises the following error:
```
1 validation error for PartialUser
username
Value error, must validate the regular expression /^[\w.@+-]+$/ [type=value_error, input_value='Rossi Mario example', input_type=str]
```

... the username `Rossi Mario example` is a valid username

Start by reproducing the failure through StagesApi().stages_invitation_invitations_create() with authentik_client 2025.10.3, then trace the PartialUser username validation that rejects “Rossi Mario example”. Done means the API client accepts this returned username without raising the validation error.

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