## Problem
When aborting a split, the code accidentally removes all other tenant
shards from the in-memory map that have the same shard count as the
aborted split, causing "tenant not found" errors. It will recover on a
storcon restart, when it loads the persisted state. This issue has been
present for at least a year.
Resolves https://github.com/neondatabase/cloud/issues/28589.
## Summary of changes
Only remove shards belonging to the relevant tenant when aborting a
split.
Also adds a regression test.