[Bug]: GitLab file and image comments are dropped from review threads #30424

Area

Pull requests (GitLab provider)

Steps to reproduce

1. On a GitLab merge request, comment on a whole file (the comment icon in a file header) or on an image in the diff. TonybynMp4/git-ansible!5 has one of each, on t3-fixtures/notes.md and t3-fixtures/logo.png.
2. Open the merge request in T3 Code and go to the Code tab.

Expected behavior

Both comments are review threads, like GitHub file-level comments. They're listed under "Conversations not on the current diff" with Resolve and Reply, and they count as unresolved findings.

Actual behavior

They only show up as plain comments in the Summary. The Code tab has no trace of them, they can't be resolved from T3 Code, and they don't count as unresolved findings.

The discussions decoder skips every position whose position_type isn't text. GitLab uses file for file comments and image for image comments, both with no line:

$ glab api "projects/TonybynMp4%2Fgit-ansible/merge_requests/5/discussions"   (root notes, non-text positions)
{"body":"Image comment on the logo.","position_type":"image","new_path":"t3-fixtures/logo.png","new_line":null}
{"body":"File comment on notes.md as a whole.","position_type":"file","new_path":"t3-fixtures/notes.md","new_line":null}

Impact

Review feedback on whole files or images is invisible to the thread workflow, so it gets missed.

Version or commit

b183048a82 (main)

Environment

Linux, web dev build, gitlab.com through glab.

Screenshots, recordings, or supporting files

On main, the Code tab shows only the line threads, nothing for the file or image comments:

Workaround

Resolve and reply to them on GitLab.