Plugin-only providers (commandcode, tencent-tokenhub, ...) are absent from
models.dev and HERMES_OVERLAYS, so resolve_provider_full returned None and
/model switches failed with "Unknown provider ..." even though the picker
lists them (CANONICAL_PROVIDERS auto-extends from the same registry).
Fall back to providers.get_provider_profile() before giving up, mapping the
profile api_mode to the ProviderDef transport.