Custom-endpoint activation writes model.key_env, but
clear_model_endpoint_credentials never popped it and the switch-clears
trigger only fired on inline keys — a pointer-only model block survived
every provider switch, routing the new provider's requests to the old
endpoint's env var. key_env/api_key_env now clear under clear_api_key;
the trigger fires on pointer-only blocks too. All key_env writers set it
after the clear, so custom-to-custom switches keep the new pointer.
Teknium
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