Auto-collapse resolved MR threads

What does this MR do and why?

For #357827

When a thread on a diff line is resolved, the discussion panel automatically collapses.

Supported scenarios:

• Overview tab: Threads on diff lines of code/text files
• Changes tab: Threads on diff lines of code/text files
• File-level discussions
• Unattached discussions (MR-level threads from Overview tab)
• Threads resolved manually via "Resolve thread" button
• Threads resolved by applying a suggestion

Not yet supported:

• Whole file discussions on image files
• Point comments on images

References

• !207153 (closed)

Screenshots or screen recordings

Before	After
Auto-collapse does not happen before this MR

How to set up and validate locally

1. Create an MR with code changes
2. Add a discussion on a line of code
3. Resolve the discussion
4. Observe the discussion panel collapses automatically
5. Click the replies toggle to expand and verify you can still unresolve

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.