[Resolute] `passwd` fails for local users on authd accounts

Attempting to use the `passwd` command as a local user, on an authd user fails on Resolute. But works on earlier releases, as demonstrated in #851

```bash
$ sudo passwd @
passwd: Provided user name is not a valid name
```

The context here is that local users should be able to change authd users' passwords, so it can be a reliable way of changing _forgotten_ passwords by an admin local user, rather than relying on device-auth flow as in https://github.com/canonical/authd/issues/369#issuecomment-4294943575

Related issues: https://github.com/canonical/authd/issues/851 , https://github.com/canonical/authd/issues/850, https://github.com/canonical/authd/issues/369

Start by reproducing the documented `sudo passwd @` failure on Resolute and compare it with the behavior described in issue #851. Trace how authd handles password changes for local administrators and define done as allowing the command to change an authd user's forgotten password without breaking earlier password flows.

Written by the indexing model from the issue text.