- Replace fragile substring check (any(kw in backend for kw in ('clickhouse', 'ch')))
with a proper _backend_normalize mapping + exact comparison.
Fixes misdetection of Greenplum (postgresql) and other backends.
- Add 'clickhousedb' to CLICKHOUSE_DIALECTS so SQL generator uses
backtick quoting and correct INSERT strategy for ClickHouse.
- Normalize _extract_dialect to map 'clickhousedb' -> 'clickhouse'
and 'greenplum' -> 'postgresql'.