Group search results by entity type in collapsible sections

Problem/Motivation

When a search matches many entities of more than one type (e.g. hundreds of nodes and content blocks), the results page is one long list with only an h3 between the entity types. It's hard to see where one type ends and the next begins, or how many matches each type has, without scrolling through everything.

Proposed resolution

Render each entity type's results in a details element:
- The summary shows the type's label with its counts, e.g. "Content block (15 entities, 42 matches)".
- Groups are open by default for small result sets and closed for large ones, above a threshold such as 20 entities, so a large search shows a compact overview first.
- The list inside each group is unchanged.

This applies to both search results and the "Content that was changed" output after a replace, since they share the scanner_results template.

Remaining tasks

- Update scanner-results.html.twig and add preprocessing for the groups.
- Add test coverage for the grouping and the open/closed behaviour.
- Review.

User interface changes

Before

After

API changes

None.

Data model changes

None.