Factoring out a common error handler method for `UnknownTopicOrPartitionException` within the `HttpAdminBridgeEndpoint`

[ppatierno]

The HttpAdminBridgeEndpoint.doGetTopic method has multiple places with code duplication when handling the UnknownTopicOrPartitionException.
The pattern seems to be the same:

if (ex.getCause() instanceof UnknownTopicOrPartitionException) {
 // send NOT_FOUND
} else {
 // send INTERNAL_SERVER_ERROR
}

We should refactor by extracting a common handleAdminError (or similar) to be used across these places.

[scholzj]

Triaged on 30.4.2026: Sounds reasonable.