Bug: `SYSTEM DROP DATABASE REPLICA` uses wrong shard identifier during scale-down of Replicated database with custom shard/replica names

Summary

When scaling down a ClickHouseInstallation that uses Replicated database engine, the operator generates an incorrect SYSTEM DROP DATABASE REPLICA command. The stale replica metadata is not cleaned from ClickHouse Keeper, causing REPLICA_ALREADY_EXISTS errors on subsequent scale-up.

Environment

• ClickHouse Operator version: latest (tested on current main)
• ClickHouse version: 25.3.2.1
• Database engine: Replicated

Steps to Reproduce

1. Create a ClickHouseInstallation with 1 shard, 1 replica using Replicated database engine:

   CREATE DATABASE myDB ON CLUSTER 'all-replicated' ENGINE = Replicated('/clickhouse/databases/myDB', '{all-sharded-shard}-{shard}', '{replica}')
   

2. Scale up to 2 replicas
3. Scale back down to 1 replica
4. Scale up again to 2 replicas

At step 4, the new replica fails with:

Code: 253. DB::Exception: There was an error on [chi-clickhouse-scw-0-1:9000]:
Code: 253. DB::Exception: Replica /clickhouse/databases/default/replicas/1-0|chi-clickhouse-scw-0-1
already exists. (REPLICA_ALREADY_EXISTS)

Root Cause

In pkg/model/chi/schemer/sql.go:244-248, the sqlDropReplica function generates:

func (s *ClusterSchemer) sqlDropReplica(shard int, replica string) []string {
    return []string{
        fmt.Sprintf("SYSTEM DROP REPLICA '%s'", replica),
        fmt.Sprintf("SYSTEM DROP DATABASE REPLICA '%d|%s'", shard, replica),
    }
}

The shard parameter is an int (0-based ShardIndex), so the generated command is:

SYSTEM DROP DATABASE REPLICA '0|chi-clickhouse-scw-0-1'

However, the Replicated database engine is created with macros for the shard identifier (e.g. {all-sharded-shard}-{shard}), which resolve to a string like 1-0. ClickHouse Keeper stores replica entries using this resolved database_shard_name (visible in system.clusters), not the integer shard index. The actual znode path is:

/clickhouse/databases/default/replicas/1-0|chi-clickhouse-scw-0-1

So the correct command should be:

SYSTEM DROP DATABASE REPLICA '1-0|chi-clickhouse-scw-0-1'

Start in pkg/model/chi/schemer/sql.go at sqlDropReplica and inspect how the shard identifier is represented when the Replicated database uses custom shard and replica names. Verify the generated SYSTEM DROP DATABASE REPLICA command against the resolved identifier shown in the issue, then reproduce the scale-down and scale-up sequence to confirm stale replica metadata is removed.