Bug: Matching built-in providers fails if specified by name rather than inferred from model.

[telackey]

If a built-in provider is specified by name via a ModelConfig would config, it will fail, because is used router.resolve_provider(config.provider) without first calling providers.load_builtins_once(), which is only called when the provider is being inferred rather than specified.

Before:

langextract.core.exceptions.InferenceConfigError:
No provider found matching: 'ollama'. Available providers can be listed with list_providers()

PR:
#331