test: failing reproducer - no error for high UID (>65535)

[lengau]

Reproducer for #259

Enterprise systems using NIS, SSSD, or Active Directory commonly assign UIDs above 65535. When map_user_uid=True with such a UID, LXD rejects it with:

Error: Failed instance creation: Failed creating instance record:
Failed initialising instance: Host id is in the range of subids

This message is cryptic. craft-providers should detect this condition and raise a clear ProviderError before calling LXD.

Failing test

test_set_id_map_raises_for_high_uid — shows that _set_id_map() currently calls lxc.config_set(raw.idmap=...) for UIDs > 65535 rather than raising a descriptive error first.

Fix

Add a check in _set_id_map() before the lxc.config_set call: if uid > 65535, raise a ProviderError with a link to documentation.

Closes #259