fix(clickhouse): preserve legacy soft-delete column casing

[floze-the-genius]

Summary

• resolve the configured soft-delete column against the physical ClickHouse destination schema before normalization
• prefer exact matches, then case-insensitive matches, so both legacy lowercase tables and newer uppercase/custom tables keep working
• apply the same resolution during destination validation

Root cause

Older PeerDB releases stored the UI default _PEERDB_IS_DELETED in mirror config, but ClickHouse table creation and normalization hardcoded the physical column as _peerdb_is_deleted. After #4365 began honoring the stored config, upgraded legacy mirrors generated inserts for the uppercase identifier even though their existing tables only had the lowercase column.

Tests

• go test ./connectors/clickhouse -skip '^TestCreateRawTableHasTTL$' -count=1
• go vet ./connectors/clickhouse
• go test -race ./connectors/clickhouse -run 'Test(ResolveSoftDeleteColumnName|BuildQuery_UsesDestinationSoftDeleteColumnCase|ProcessTableComparison_WithResolvedSoftDeleteColumnCase)$' -count=1

Fixes #4596

[CLAassistant]

All committers have signed the CLA.

[pfcoperez]

Thanks for your contribution! I've dropped a comment about the implementation detail.

[floze-the-genius]

Narrowed the compatibility behavior in ac5fe774 as requested:

• exact destination-column matches still win;
• fallback resolution is now limited to the two known equivalent defaults, _PEERDB_IS_DELETED and _peerdb_is_deleted;
• mixed-case variants and custom column names remain strict, so my_deleted will not resolve to MY_deleted.

Added positive tests for both legacy directions and negative tests for mixed/default and custom-case variants. Targeted ClickHouse tests and go vet ./connectors/clickhouse pass.