The original fix preserved full IDs only when config_provider == 'custom',
which broke existing tests expecting prefix-stripping for known namespaces
like 'openai/' and 'google/'.
The correct heuristic: strip the prefix only when it is a known provider
namespace (i.e. prefix in _PROVIDER_MODELS — 'openai', 'google', 'anthropic',
etc.). Unknown prefixes like 'zai-org' are intrinsic to the model ID and must
be preserved. This satisfies both the DeepInfra use case (#548) and the
existing #433 regression tests.
dc2334c5a3