Support add_labels / remove_labels in update Merge Request

[mhagnumdw]

Summary

Adds withAddLabels(...) and withRemoveLabels(...) to MergeRequestParams, mapping to the add_labels and remove_labels form parameters of the GitLab Update Merge Request endpoint: https://docs.gitlab.com/api/merge_requests/#update-a-merge-request

Motivation

Currently MergeRequestParams only exposes withLabels(...), which sends the full labels list and replaces the merge request's labels entirely. Adding or removing a single label therefore requires fetching the current label set and re-sending it.

The GitLab API already supports incremental label changes via add_labels and remove_labels only on the update endpoint: https://docs.gitlab.com/api/merge_requests/#update-a-merge-request

Changes

• MergeRequestParams: two new fields (addLabels, removeLabels) and serialization wired into the update-only branch of getForm(boolean isCreate)
• TestMergeRequestParams: add some tests

The change is strictly additive - existing behavior of withLabels(...) is unchanged.

Usage

mergeRequestApi.updateMergeRequest(projectIdOrPath, mrIid,
    new MergeRequestParams()
        .withAddLabels(List.of("bug"))
        .withRemoveLabels(List.of("wontfix")));

ps: I'm already using these changes in my application and everything is fine.