DEV: refactor custom-user-selector to use DMultiSelect

[tyb-talks]

This replaces the jquery-based autocomplete in user-selector with the DMultiSelect component (similar to what was done in discourse/discourse#34685).

Existing behaviour:

text-field-user-selector.mov

Caveat:
I'm unsure how the higher level wizard components depend on the user-selector being a TextField-based component.. If we can't use DMultiSelect here, the alternative would be applying the DAutocomplete modifier directly to the text field (example: discourse/discourse#34131).

Key Changes

• Converted custom-user-selector from Classic Ember component to Glimmer component, passing in required properties explicitly from parent Classic component

• Replaced jQuery autocomplete with DMultiSelect for modern UI patterns

• Maintained backward compatibility with existing @onchangecallback interface

• Automatic user exclusion: DMultiSelect automatically filters selected users from search results (eliminates
  custom excludedUsernames() logic)

• Built-in keyboard navigation and accessibility features

• Search results display: Users show avatars + username/name, groups show group icon + name

• Selected items display: Plain text (username for users, name for groups) without icons - matches original
  behavior

• Modern dropdown positioning using FloatKit with configurable placement options

• Removed Legacy Options

  • allowAny: Not needed with DMultiSelect (was jQuery autocomplete-specific)
  • updateData: Replaced by DMultiSelect's automatic reactive updates
  • Template functions: Replaced with Handlebars named blocks (<:selection>, <:result>)

Testing

New behaviour:

d-multi-select-user-selector.mov

[tyb-talks]

this is meant to do the same thing as the previous 2-way binding from TextField and assumes there's some kind of dependency on this.field.value for this wrapper component. I may be wrong here and this is not necessary.

[angusmcleod]

@tyb-talks Amazing, thank you.

[tyb-talks]

@tyb-talks Amazing, thank you.

🙇

Could I also trouble you to trigger the CI? (I don't see an option for forcing a run and it seems stuck)

And I don't really have the context to do a comprehensive manual QA on the wizard, if you have something scaffolded up on a test site or locally, it would be safer to do a check on your end. The plugin's test suite fails on a few tests locally which seem unrelated to this change so I suspect my local setup isn't fully compatible with this plugin.

[tyb-talks]

@tyb-talks Amazing, thank you.

🙇

Could I also trouble you to trigger the CI? (I don't see an option for forcing a run and it seems stuck)

And I don't really have the context to do a comprehensive manual QA on the wizard, if you have something scaffolded up on a test site or locally, it would be safer to do a check on your end. The plugin's test suite fails on a few tests locally which seem unrelated to this change so I suspect my local setup isn't fully compatible with this plugin.

@angusmcleod in case you missed the above :)