Fix flaky work item spec

What does this MR do and why?

We need to wait for the description widget to load before saving otherwise we'd get an error.

See sample non-headless run:

Screen_Recording_2025-07-29_at_9.39.09_PM

References

Related to #557769 (closed)

How to set up and validate locally

bin/rspec './spec/features/boards/sidebar_spec.rb[1:2:1:6:1]'

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