Only the --provider path handed user_providers/custom_providers to
resolve_alias, so the provider-identity comparison behind the
current-provider preference could not see legacy custom_providers
entries on the implicit path (/model <id>) or the authenticated-provider
fallback. On custom:corp-llm, /model shared-model picked another
provider's alias for the same model id and switched to its base_url.
Pass both provider maps on every resolve_alias call site. The two
existing resolve_alias stubs in test_ollama_cloud_auth.py accept the
extra positional args; their assertions are unchanged.
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