Contexts are typically used via a MelleaSession. In that setting, contexts are "bound" to a specific model_id. When we know the model_id, we can lookup the max context length and then use this context length to determine when the Context's sliding window needs to move. We should implement a DynamicLinearContext that provides this behavior, or maybe even just change the LinearContext to do this dynamic window management based upon models' max context length.