Advanced merge request search should allow searching in merge request comments

Problem to solve

It's hard to remember if you commented on something or if it was in the description. Users would like to see merge requests with matching comments in the merge request search results

Proposal

Search the notes (comments) index when searching the merge requests index. Limit the notes search to return only comments on merge requests and only 100 records.

Release notes

This feature adds the ability to search within merge request comments when searching for GitLab merge requests using advanced search, with results in the merge request scope containing merge requests that match the search term in both merge request descriptions and merge request comments. This enhancement addresses a usability problem where users previously experienced a fragmented search workflow, having to search merge requests and comments separately, which made it difficult to efficiently find relevant information and required switching between different search scopes to get a complete picture.