[release-20.0] Fix `vtexplain` not handling `UNION` queries with `weight_string` results correctly. (#16129) #41235

Job	Run time
Local example using consul on ubuntu-22.04	31m 49s
Local example using consul on ubuntu-22.04	10m 8s
Local example using etcd on ubuntu-22.04	12m 36s
Local example using etcd on ubuntu-22.04	12m 36s
Local example using zk2 on ubuntu-22.04	13m 52s
Local example using zk2 on ubuntu-22.04	13m 52s
	1h 34m 53s

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