refactor: delegate transaction handling to plugin drivers

[datlechin]

Summary

• Each plugin driver now owns its transaction SQL instead of the adapter hardcoding BEGIN/COMMIT/ROLLBACK
• PluginDriverAdapter delegates to pluginDriver.beginTransaction() / commitTransaction() / rollbackTransaction()
• MySQL overrides with START TRANSACTION, MSSQL with BEGIN TRANSACTION, Oracle with no-op (implicit transactions)
• MongoDB marks supportsTransactions: false (Redis already has MULTI/EXEC support)
• Removed DatabaseType.beginTransactionSQL — transaction knowledge lives in plugins
• Removed default transaction implementation from DatabaseDriver extension
• All call sites (DatabaseManager, coordinators, ImportService) use protocol methods instead of raw SQL strings

Test plan

• Build succeeds
• All tests pass
• Verify MySQL connection can save row edits (uses START TRANSACTION)
• Verify PostgreSQL transaction wrapping still works (uses BEGIN)
• Verify SQL Server transaction wrapping works (uses BEGIN TRANSACTION)
• Verify Oracle operations work without explicit BEGIN
• Verify Redis MULTI/EXEC still works
• Verify ClickHouse/MongoDB operations work without transactions

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