Test `write_multiple` cache can be read back by `read_multiple`

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

Fix #438789 (closed)

Implement this part of original discussion:

"what we mean to write gets written, as shown by our ability to read it back out an assert equivalence"

References

Screenshots or screen recordings

N/A

How to set up and validate locally

• bundle exec rspec spec/lib/gitlab/discussions_diff/highlight_cache_spec.rb

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 #438789 (closed)