KAFKA-18573: Close verification key resolver in BrokerJwtValidator

[FrankYang0529]

Why

BrokerJwtValidator#configure obtains a
CloseableVerificationKeyResolver from
VerificationKeyResolverFactory.get() but never closes it, so the JWKS
refresh thread of RefreshingHttpsJwksVerificationKeyResolver leaks.

How

• BrokerJwtValidator
  • Keeps a reference to the resolver obtained from the factory,
    configures it through the ref-counting wrapper, and releases it in
    close().
  • An injected resolver is not closed. Its lifecycle is managed by the
    code that supplied it.
• VerificationKeyResolverFactory
  • create() no longer configures the resolver; initialization is
    driven by the wrapper's configure().
  • RefCountingVerificationKeyResolver removes its cache entry when
    the last reference is closed, so a subsequent get() with the same
    configuration builds a fresh resolver instead of returning the closed
    one.