MR Code review: Allow "Prevent merge unless all threads are resolved" to exclude Duo comments

Problem to solve

Currently, the Prevent merge unless all threads are resolved setting treats all comments equally - whether they come from human reviewers or from GitLab Duo AI. This creates a limitation for teams who want to:

• Require resolution of human reviewer feedback before merging
• Use Duo Code Review for advisory feedback without blocking merges
• Maintain strict human review requirements while benefiting from AI assistance

When Duo Code Review leaves comments on a merge request, those comments currently block merging just like human comments do, even though teams may want different policies for AI-generated feedback versus human reviewer feedback.

User experience goal

As a project maintainer, I want to configure my merge request settings so that only unresolved discussions from human reviewers block merging, while Duo comments remain advisory and don't prevent merges. This allows teams to benefit from AI code review insights without adding AI comments to merge blocking requirements.

Proposal

Add a configuration option to the "Merge request approvals" settings that allows projects to exclude Duo comments from the "prevent merge if discussions unresolved" requirement.