MYSQL-OBJ-006: Visible and invisible index management

Fixture

• init.sql creates obj006_index_test with primary key, unique, ordinary, and composite indexes.
• On MySQL 8.0+, idx_name is made INVISIBLE via ALTER INDEX.
• grants.sql grants SELECT, INDEX, ALTER to the test user.
• cleanup.sql drops the test table.

Verification

1. Connect with the test user.
2. Open the table in the table editor.
3. Verify the index list shows VISIBLE/INVISIBLE for each index.
4. Toggle idx_name from INVISIBLE to VISIBLE and preview the SQL.
5. Execute and refresh — the index should show VISIBLE.
6. Toggle uk_code to INVISIBLE and verify the SQL previews correctly.
7. Verify the primary key row does not offer a visibility toggle.
8. On MySQL 5.7, the visibility column should not appear.