Use locale-aware collation for attendee list sorting

[dd32]

Summary

• Adds utf8mb4_unicode_520_ci collation to the ORDER BY clause when the [camptix_attendees] shortcode sorts by title
• The default utf8mb4_unicode_ci collation uses Unicode 4.0 sorting rules, which incorrectly sorts characters like Polish Ł (after Z instead of with L)
• Uses posts_orderby filter with suppress_filters => false as discussed in the issue comments by @dd32 and @pkevan

Changes

• camptix/addons/shortcodes.php: Added suppress_filters => false to get_posts() args, and a temporary posts_orderby filter that adds the COLLATE clause when sorting by title

Implementation details

• The filter is added just before the query and removed immediately after, so it only affects this specific query
• The collation override only applies when orderby is title (the default)
• Per @dd32: SELECT post_title FROM wc_posts WHERE post_type = 'tix_attendee' ORDER BY post_title COLLATE 'utf8mb4_unicode_520_ci' ASC produces correct sorting

Test plan

• Existing CampTix tests pass (63 tests, 80 assertions)
• Verify Polish characters (Ł, Ś, Ź, etc.) sort correctly on a site with Polish attendee names
• Verify sorting still works correctly for ASCII-only attendee names

Fixes #1310

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