Migrate read-only work item sidebar cases to MSW

What does this MR do and why?

https://gitlab.com/gitlab-org/plan-stage/plan-engineering/-/work_items/1417

This is the fifth slice of an ongoing series that migrates work item detail sidebar widgets from Capybara feature specs to MSW frontend integration specs.

It adds ee/spec/frontend/integration/work_items/details/read_only_sidebar_spec.js. The spec opens the work item drawer with a payload whose permissions are off, and checks the sidebar still renders its attribute values (status, assignees, labels) but offers no edit button anywhere in it. It runs under both states of the workItemFeaturesField flag.

Worth calling out: the deleted guest example asserted the absence of the selector .block.labels .js-sidebar-dropdown-toggle. That selector is markup from the old issue sidebar, which the work item sidebar does not render at all, so the example passed whether or not a guest could actually edit anything. The replacement spec asserts there is no edit button in the sidebar, which is the thing the example meant to check in the first place.

References

• Parent issue: https://gitlab.com/gitlab-org/plan-stage/plan-engineering/-/work_items/1417
• Earlier slices in the series:
  • Weight: !252836 (merged)
  • Iteration: !253470 (merged)
  • Health status: !254556 (merged)
  • Status: !255742 (merged)

How to set up and validate locally

1. Run the new MSW integration spec:

   yarn jest:integration ee/spec/frontend/integration/work_items/details/read_only_sidebar_spec.js

2. Run the trimmed Capybara specs to confirm the remaining examples still pass:

   bundle exec rspec ee/spec/features/work_items/detail/work_item_detail_spec.rb spec/features/work_items/issues/issue_sidebar_spec.rb

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