Redesign recommended reviewers sidebar group

What does this MR do and why?

Rework the recommended reviewers group in the MR reviewers sidebar so it reads as a single collapsible group rather than per-rule sub-lists.

The old layout grouped suggestions under a bold rule heading per approval rule, which competed visually with the assigned reviewers and gave no way to get the group out of the way. Each suggestion is now a two-line row (name plus role) under one lightweight "Recommended" toggle. Collapsing the group persists per user via localStorage and shows the reviewers as a stacked avatar group, keeping the sidebar compact on MRs the user is not acting on. Recommended avatars use a dashed, unfilled treatment so they are not mistaken for assigned reviewers.

The header is a button exposing aria-expanded/aria-controls, and both the user hover card and the matching-files tooltip are preserved on hover and keyboard focus.

Highlights

• Single collapsible Recommended group with a two-line row per reviewer (name + role), replacing the per-approval-rule sub-headings.
• The role line keeps the dotted-underline "Matching files" popover; the user hover card is preserved on the name/avatar. Both open on hover and keyboard focus.
• Recommended avatars use a dashed, unfilled treatment to distinguish them from assigned reviewers, while the availability status dot stays legible.
• Collapsed state hides the rows and shows a stacked avatar group (GlAvatarsInline, up to 3 + overflow) with a single accessible label.
• Collapse state persists per user across MRs via LocalStorageSync (key suggested_reviewers_collapsed).
• Adding a recommended reviewer still moves them into the assigned list and drops them from this group; the group renders nothing when empty.

References

Follow-up polish to the recommended reviewers UI in the MR reviewers sidebar.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. On a GDK with an EE license and AI suggested reviewers enabled, open a merge request that has recommendations.
2. In the sidebar Reviewers section, confirm the Recommended group:
   • Expanded by default; each reviewer is a two-line row (name + role with dotted underline).
   • Hover/focus a role to see the "Matching files" popover; hover/focus a name/avatar for the user card with the reason.
   • Click the header to collapse: rows hide and up to 3 dashed avatar chips (then +N) appear.
   • Reload another MR and confirm the collapse state persisted.
   • Add a recommended reviewer and confirm they move to the assigned list and leave the group; adding the last one renders nothing.

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