When an alias like 'claude' can't be resolved on the current provider,
_resolve_alias_fallback() tries other providers. Previously it hardcoded
('openrouter', 'nous') — so '/model claude' on z.ai would resolve to
openrouter even if the user doesn't have openrouter credentials but does
have anthropic.
Now the fallback uses the user's actual authenticated providers (detected
via list_authenticated_providers which is backed by the models.dev
in-memory cache). If no authenticated providers are found, falls back to
the old ('openrouter', 'nous') for backwards compatibility.
New helper: get_authenticated_provider_slugs() returns just the slug
strings from list_authenticated_providers().
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