Merge Request with conflict sometines does not show the conflict in the diff (missing conflict_type)

Problem

Sometimes the server responds with has_conflicts: true but conflict_type: null. This prevents us from being able to accurately display the conflict state.

When both fields are present but in this inconsistent state, the UI cannot determine what type of conflict exists, making it impossible to show users the appropriate conflict information or resolution options.

Example: gitlab-com/content-sites/handbook!17166 (comment 2948044618)

Proposal

Always provide a conflict_type when has_conflict is true.