Phase 1, Task 1.3. Mirrors the existing register_image_gen_provider
pattern (plugins.py:531) — wrong-type or duplicate-name registrations
log at WARNING and silently return rather than raising, so a misbehaving
auth plugin cannot crash the host.
Deviation from plan: the plan's draft raised TypeError on non-provider
input; switched to silent-warn to match the established image_gen
convention. Test updated to match.
