test/cluster: wait for ready CQL in cross-rack merge test

test_tablet_merge_cross_rack_migrations() starts issuing DDL immediately
after adding the new cross-rack nodes. In the failing runs the driver is
still converging on the updated topology at that point, so the control
connection sees incomplete peer metadata while schema changes are in
flight.

That leaves a race where CREATE TABLE is sent during topology churn and
the test can surface a misleading AlreadyExists error even though the
table creation has already been committed. Use get_ready_cql(servers)
here so the test waits for inter-node visibility and CQL readiness
before creating the keyspace and table.

Fixes: SCYLLADB-1635

Closes scylladb/scylladb#29561

test/cluster/test_tablets_merge.py

@@ -334,7 +334,7 @@ async def test_tablet_merge_cross_rack_migrations(manager: ManagerClient, racks)
         rack = f'rack{rack_id+1}'
         servers.extend(await manager.servers_add(3, config=config, cmdline=cmdline, property_file={'dc': 'mydc', 'rack': rack}))
 
-    cql = manager.get_cql()
+    cql, _ = await manager.get_ready_cql(servers)
     ks = await create_new_test_keyspace(cql, f"WITH replication = {{'class': 'NetworkTopologyStrategy', 'replication_factor': {rf}}} AND tablets = {{'initial': 1}}")
     await cql.run_async(f"CREATE TABLE {ks}.test (pk int PRIMARY KEY, c blob) WITH compression = {{'sstable_compression': ''}};")