refactor(driver-sql): move the NULL-safe unique pre-flight and the declared-index sync into sql-driver-schema-sync.ts (cut 9)

The bodies of SqlDriver#applyNullSafeUniquePreflight, #syncDeclaredIndexes
and #createNullSafeUniqueIndex move unedited into sql-driver-schema-sync.ts
through the existing SchemaSyncHost, widened by the three members they read
that it did not list (createHashShadowUniqueIndex, createNullSafeUniqueIndex,
explainUnkeyableTextColumn). The module-level formatDuplicateGroups moves with
its only two callers and stays module-local. Each class method keeps its
docblock and signature less async.

Claude-Session: https://claude.ai/code/session_01EcqJz48p6ineaiedw6SCaX
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude