allocator: domain store list is constructed using all stores passed in

`bestStoreToMinimizeDelta` selects a store to move load to from the given existing store. The candidate store list it receives is filtered to only be valid, equally diverse and constraint matching replacements.

https://github.com/cockroachdb/cockroach/blob/0495342e81d473901278993f6162b1d78b685bc1/pkg/kv/kvserver/allocator/allocatorimpl/allocator_scorer.go#L1187-L1187

The domain is constructed to be candidates+existing and is used for calculating the mean load value. The mean load value then determines if the max load thresholds. The max load threshold is used to possibly return early if the existing store isn't above that threshold. The result is, if the mean is incorrectly using a different domain than the candidates+existing, `bestStoreToMinimizeLoad` may erroneously return `existingNotOverfull` and no rebalance target.

The domain store list is not constructed using the domain currently, instead it uses the `storeDescMap` which doesn't guarantee to only include the domain stores (lease transfers don't). This is causing the above issue.

https://github.com/cockroachdb/cockroach/blob/0495342e81d473901278993f6162b1d78b685bc1/pkg/kv/kvserver/allocator/allocatorimpl/allocator_scorer.go#L1218-L1223

Jira issue: CRDB-25561