MLXLanguageModel availability always returns .available even when model is not loaded

[noorbhatia]

MLXLanguageModel declares typealias UnavailableReason = Never, which means its availability property always returns .available via the protocol's default implementation. There's no way to check whether the model has actually been downloaded and loaded into memory.

[mattt]

@noorbhatia Thanks for reporting. I just merged a fix with #138.