Preprocess hook implementation of views_view_grouping never triggering

Problem/Motivation

hook_preprocess_views_view_grouping() is never invoked for single-level grouping, regardless of the display type (page, block, etc.).

The root cause is in StylePluginBase::renderGroupingSets(). A conditional branch decides which theme hook to use for each grouping set:

• Branch A (is_array($row) && isset($row['group'])): creates a #theme => views_view_grouping render array. Only reached when the current set's rows are themselves nested grouping sets (i.e. with 2+ grouping levels, for non-leaf levels only).
• Branch B (else): calls renderRowGroup(), which uses the style's own theme (e.g. views_view_unformatted). Always taken for single-level grouping, and for the leaf level of any multi-level grouping.

Because views_view_grouping is never rendered in these cases, hook_preprocess_views_view_grouping() is never triggered.

With 2 grouping levels, the hook fires once (for level 0). Level-1 titles are accessible indirectly via $variables['content'][x]['#title'] within the level-0 hook call, but views_view_grouping is never properly invoked for the leaf level.

Steps to reproduce

1. Create a view with any display (page or block).
2. Configure the style (e.g. Unformatted list) with one grouping field.
3. Implement hook_preprocess_views_view_grouping() in a custom module.
4. The hook is never called.

Proposed resolution

Two changes are needed:

1. In StylePluginBase::renderGroupingSets(), when grouping is configured and branch B is taken (leaf rows), always build a #theme => views_view_grouping render array instead of delegating directly to renderRowGroup().
   Rows should be pre-rendered with the row plugin first, as currently done.
2. In ViewsThemeHooks::preprocessViewsViewGrouping(), detect whether $variables['rows'] contains nested grouping sets or already-rendered row render arrays:
   • nested sets → existing recursive renderGroupingSets() call
   • pre-rendered rows → wrap them with $style->themeFunctions() directly (no new public API needed)

A change record will be needed: for single-level grouping, the group title currently rendered by views_view_unformatted (inside an <h3>) moves to views_view_grouping (which currently outputs a bare {{ title }}).

Note: template_preprocess_views_view_grouping() is deprecated in Drupal 11.3.0 and removed in 12.0.0. The initial preprocess is now registered via the 'initial preprocess' key in hook_theme(), pointing to ViewsThemeHooks::preprocessViewsViewGrouping() — which is the method to update.

Remaining tasks

• [x] Write a patch
• [x] Write tests
• [ ] Review the MR
• [x] Write a change record : https://www.drupal.org/node/3592452

API changes

hook_preprocess_views_view_grouping() will now be invoked for all grouping levels, including single-level grouping and leaf levels of multi-level grouping.

BC breaks

For single-level grouping, the HTML output changes: the group title moves from views_view_unformatted to views_view_grouping. Sites overriding views-view-grouping.html.twig or implementing hook_preprocess_views_view_grouping() may need to update accordingly.