Fibre: only register the host if a validator is bonded

This makes me think that there is still a way to state bloat using the host registration. Anyone can create a huge number of validators, even if unbonded, and register their hosts and just leave them in the state. These will never be bonder and will never be jailed and will bloat the state. Also, they would increase the lookup time where we iterate over them.

Let's create an issue to address this issue subsequently. We should only register a host if the validator is bonded to reduce the impact.

_Originally posted by @rach-id in https://github.com/celestiaorg/celestia-app/pull/7457#discussion_r3459354506_