isClosed property

Whether close has already been called.

Core's embedder cache is the caller that needs this. It evicts on the close listener, which means it trusts an implementation to fire one — and a model that never does, or that was already closed before the cache took custody, would otherwise be handed to every later caller, whose first generateEmbedding throws. This lets the cache check instead of trust.

Defaulted to false rather than abstract: it reproduces exactly what the cache assumed before it existed, so an implementation that does not answer behaves as it does today. implements does not inherit a default body, so those break — which is the same break this release already takes for activeBackend, not a second one.