fix(gitlab): use diff_stats instead of changes_count for correct MR s…

[bujjibabukatta]

Problem

GitLab MR size is incorrectly inflated when a branch has multiple
sequential PRs. The changes_count field from GitLab's API returns
cumulative line changes across all commits on the branch including
previous PRs, not just the current MR's diff.

Root Cause

GitLab's changes_count sums all commits on the source branch.
For multi-part PRs this means PR #3 shows 9292 lines changed
instead of the actual 6523 lines in that PR alone.

Fix

Use GitLab's diff_stats field (via include_diff_stats=true
query param) which returns additions/deletions relative to the
merge base — only what this PR itself introduces.

Changes

• backend/plugins/gitlab/tasks/mr_extractor.go — added DiffStats
  struct to MergeRequestRes and mapped Additions/Deletions
  in convertMergeRequest
• backend/plugins/gitlab/models/mr.go — added Additions and
  Deletions fields to GitlabMergeRequest
• backend/plugins/gitlab/tasks/shared.go — added
  include_diff_stats=true query param to GetQuery
• backend/plugins/gitlab/models/migrationscripts/ — added migration
  script and registered it

Fixes #8888