Closes #19613.

Adds VISIBLE / INVISIBLE support (MySQL 8.0+) to index creation and editing.

• Index model gets isVisible(), populated from the Visible column of
  SHOW INDEXES and from form POST.
• A Visibility dropdown shows up in the index form's advanced options.
• The index list (standalone, structure tab, data dictionary) gains a Visible
  column.
• Indexes::getSqlQueryForIndexCreateOrEdit() emits the visibility via a
  separate ALTER TABLE ... ALTER INDEX x VISIBLE/INVISIBLE statement
  appended to the main ALTER. An inline keyword on the compound
  DROP INDEX, ADD INDEX is unreliable: MySQL silently ignores VISIBLE in
  certain option positions, so the new index inherits the dropped index's
  visibility. The separate ALTER INDEX always applies cleanly.
  IndexesController runs the multi-statement SQL via tryMultiQuery.
• Primary keys never get the visibility keyword (MySQL rejects an invisible
  primary key).
• Tests cover model parsing, SQL emission for create/edit/visible/invisible/PRIMARY.

Verified end-to-end against MySQL 8.0.46 (create + both edit directions).

Signed-off-by: predictor2718 245527909+predictor2718@users.noreply.github.com