AuthInterceptor constructor
Creates an interceptor that resolves headers per run/attempt and threads
the result onto the outgoing request. headers is invoked each time the
chain reaches this interceptor; its returned map is merged into the request
headers under transportHeadersKey (protocol headers always win). A throw
from headers surfaces as RunErrorEvent(BusinessError(businessAuth)).
Implementation
AuthInterceptor({required Future<Map<String, String>> Function() headers})
: _headers = headers;