Improve clickhouse validation hygiene

[jmqd]

• The ClickHouse validator distinguishes five failure modes (URL invalid, user empty, database empty, connection failed, database not found) where today everything lumps into one generic "Connection Failed" message.
• The validator rejects non-HTTPS URLs.
• Added ClickHouseClient::new_without_database and ClickHouseClient::database_exists(name) so the validator can probe connectivity and database existence independently -- mirrors BigQuery's new_with_key / dataset_exists pair.

Closes ETL-634.

[coveralls]

coverage: 72.894% (+0.03%) from 72.868% — jm/clickhouse-validation-hygiene into main