Prevent silent bugs where a CheckerDefinition with an unset ID
would be registered under the empty string key, becoming unfindable
and potentially colliding with other empty-ID registrations.
RegisterChecker and RegisterObservationProvider previously overwrote
existing entries silently, which lets a misconfigured plugin shadow a
built-in (or another plugin) without any signal. Refuse the duplicate
and keep the existing entry instead.
RegisterExternalizableChecker now performs the dedup check before
appending the "endpoint" AdminOpt, so a rejected re-registration no
longer mutates the live definition.
