Draft: Add PUT & DELETE discussion endpoint for commit Rapid Diffs

What does this MR do and why?

References

Adds the PUT and DESTROY discussions endpoint for Rapid Diffs commits.

PUT: /discussions/:note_id
DELETE: /discussions/:note_id

Screenshots or screen recordings

Before	After

How to set up and validate locally

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.

Related to #571466