New MV Implementation - Testing & Compatibility Coverage


We need to validate backward compatibility and test coverage for new and existing MV workflows.

### Description

* Create test models for both new and legacy materialization modes.

* Add integration tests for:

  * Multiple MVs on one target table.

  * Backfill and schema drift handling.

  * Clustered setups (ON CLUSTER).

  * Validate existing user models don’t break.