Add Unit Test to Validate Schema Version Check Removal in TiCDC

[wlwilliamx]

Description

This issue focuses on adding a unit test to ensure the correctness of TiCDC’s DDL processing logic after the removal of the schema version check in #11733. Previously, TiCDC discarded DDL jobs if their schema versions were smaller than the current schema version, even when their commit timestamps were valid. To address this issue, the schema version check was removed, allowing DDL jobs with out-of-order schema versions to be processed as long as their commit timestamps are valid.

[wlwilliamx]

/affects 6.5 7.1 7.5 8.1 8.5