storcon: reproduce shard split issue (#11290)

## Problem

Issue https://github.com/neondatabase/neon/issues/11254 describes a case
where restart during a shard split can result in a bad end state in the
database.

## Summary of changes

- Add a reproducer for the issue
- Tighten an existing safety check around updated row counts in
complete_shard_split

storage_controller/src/service.rs

@@ -5456,6 +5456,8 @@ impl Service {
             }
         }
 
+        pausable_failpoint!("shard-split-pre-complete");
+
         // TODO: if the pageserver restarted concurrently with our split API call,
         // the actual generation of the child shard might differ from the generation
         // we expect it to have.  In order for our in-database generation to end up