fa11b11cf5
resolve_custom_provider() previously returned api_key_env_vars=() for every custom provider entry, silently dropping the configured key_env field. This caused 401 errors for any custom provider that required an API key via environment variable (e.g. Xiaomi MiMo Token Plan, self-hosted OpenAI-compatible servers). The key_env field is already documented in _VALID_CUSTOM_PROVIDER_FIELDS and normalized by normalize_custom_provider_entry(), so this was just an oversight in the ProviderDef construction. Also adds a regression test that verifies key_env is properly propagated into the resolved ProviderDef.