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

test_runner/fixtures/neon_fixtures.py

@@ -1725,6 +1725,8 @@ class LogUtils:
             log.warning(f"Skipping log check: {logfile} does not exist")
             return None
 
+        log.info(f"Checking log {logfile} for pattern '{pattern}'")
+
         contains_re = re.compile(pattern)
 
         # XXX: Our rust logging machinery buffers the messages, so if you
@@ -2618,10 +2620,13 @@ class NeonProxiedStorageController(NeonStorageController):
         self.running = False
         return self
 
+    def instance_log_path(self, instance_id: int) -> Path:
+        return self.env.repo_dir / f"storage_controller_{instance_id}" / "storage_controller.log"
+
     def assert_no_errors(self):
         for instance_id in self.instances.keys():
             assert_no_errors(
-                self.env.repo_dir / f"storage_controller_{instance_id}" / "storage_controller.log",
+                self.instance_log_path(instance_id),
                 "storage_controller",
                 self.allowed_errors,
             )
@@ -2629,7 +2634,14 @@ class NeonProxiedStorageController(NeonStorageController):
     def log_contains(
         self, pattern: str, offset: None | LogCursor = None
     ) -> tuple[str, LogCursor] | None:
-        raise NotImplementedError()
+        for instance_id in self.instances.keys():
+            log_path = self.instance_log_path(instance_id)
+            checker = LogUtils(log_path)
+            found = checker.log_contains(pattern, offset)
+            if found is not None:
+                return found
+
+        return None
 
 
 @dataclass