[openfeature] allow recovering from bad init - #474

The adapter had no shutdown mechanism. close() now reverts it to its
uninitialized state, so the next evaluation initializes it again, per
section 2.5 of the OpenFeature provider spec. It first awaits an
initialization that is still in flight, so the client that initialization
produces is disposed rather than leaked, and it is idempotent.

Provider disposal is opt-in through the new onClose option, because the
adapter cannot do it on its own: an OpenFeature client has no close of
its own, and shutting down providers means closing them on the global
OpenFeature API, which would also affect providers this adapter never
registered.

Follow-up to #474, which fixed #473.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>