Subprocess environment construction (originator override, API key forwarding).
@spec base_overrides(String.t() | nil, String.t() | nil) :: map()
@spec maybe_put(map(), String.t(), String.t() | nil) :: map()
@spec maybe_put_openai_base_url(map(), String.t() | nil) :: map()
@spec to_charlist_env(map()) :: [{String.t(), String.t()}]