Auto-collapse fully resolved MR discussion threads

What does this MR do and why?

For #357827

When every (or the only) discussion on a line of code is resolved, the entire discussion panel will be collapsed.

Screenshots or screen recordings

Before	After
Screencast_from_2025-07-15_23-37-26	Screencast_from_2025-07-15_23-33-09

How to set up and validate locally

1. Have an MR with a line of change
2. Open 1 or more discussions on the line of code
3. Resolve all of the discussions on the line

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.