feat(mysql): invisible column management for MySQL 8.0.23+

[Aias00]

Summary

• Add visible field to TableColumn domain model and detect "INVISIBLE" from the EXTRA column in information_schema.columns
• Generate INVISIBLE keyword in column DDL via MysqlColumnTypeEnum.buildCreateColumnSql()
• Frontend: add a "Visible" column to the column editor with a VISIBLE/INVISIBLE toggle, gated to MySQL

Closes #2571

Test plan

• MySQL 8.0.23+: open a table with an invisible column — verify INVISIBLE state is shown
• Toggle a column from INVISIBLE to VISIBLE — verify SQL preview includes MODIFY COLUMN ... VISIBLE
• Create a new invisible column — verify SQL includes INVISIBLE
• Verify SELECT * omits invisible columns (explain in UI tooltip)
• MySQL 5.7 and 8.0.22 or earlier: verify the visibility control does not appear
• mvn -B compile passes for the MySQL plugin module
• eslint passes with zero warnings

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[openai0229]

Follow-up review: changes requested before merge.

P1: the column metadata path calls FIELD_EXTRA.contains(...) without handling a NULL value. MySQL returns NULL for this metadata field on ordinary columns, so opening or refreshing column metadata can throw a NullPointerException. Please normalize nullable metadata before checking it, add a fixture/test for NULL EXTRA.