[6.x] Improve the UX of taggable comboboxes

[duncanmcclean]

This pull request improves the UX of taggable comboboxes.

• The typed value now always appears as an Add "..." option at the top of the dropdown, alongside any matching options, rather than only when nothing matched. Since typing highlights the first option, it's always clear what pressing enter will do.
• Tabbing into a taggable combobox now opens the dropdown, so the available options are visible right away.
• Fixes an issue where tabbing onto a combobox focused the wrapping trigger element first, requiring a second tab to reach the search input.
  • This was happening because both the trigger and the search input were in the tab order.
  • I've fixed it by taking the trigger out of the tab order whenever the search input is visible.
• Fixes an issue where pressing enter after arrowing to an option would add the raw typed text instead of the highlighted option.
  • This was happening because the enter handler always committed the typed text, running after Reka had already selected the highlighted option and overriding its selection.
  • I've fixed it by only committing the typed text when no option is highlighted.
• Fixes an issue where adding a custom validation rule in the rule builder would silently fail in development builds.
  • This was happening because defineExpose unwraps refs, so the builder's added handler was reading searchQuery.value as undefined and throwing.
  • I've fixed it by reading the unwrapped value from the combobox ref.

This PR also renders the add option label in the relationship select and formatting locales fieldtypes, since they provide their own option templates.

Before

CleanShot.2026-08-24.at.14.50.51.mp4

After

CleanShot.2026-08-24.at.14.55.15.mp4