Filter flow_trigger_form flow listbox by search term

Phil Hughesrequested to merge
ph/flowTriggerListboxSearch into master

What does this MR do and why?

The listbox passed searchable, so it rendered a search box, but it had no @search handler. GlCollapsibleListbox renders items verbatim and only emits the term, so typing in the box did nothing. This changes filters the options with fuzzaldrin-plus.

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Screenshot_2026-09-18_at_17.06.39 Screenshot_2026-09-18_at_17.10.15

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