storcon: fix split aborts removing other tenants (#11837)

## 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.

storage_controller/src/service.rs

@@ -5181,7 +5181,8 @@ impl Service {
             }
 
             // We don't expect any new_shard_count shards to exist here, but drop them just in case
-            tenants.retain(|_id, s| s.shard.count != *new_shard_count);
+            tenants
+                .retain(|id, s| !(id.tenant_id == *tenant_id && s.shard.count == *new_shard_count));
 
             detach_locations
         };