## Description When a merge request is created targeting a branch that doesn't exist, the MR shows no diffs and provides confusing error states without clearly indicating the root cause. ## Steps to Reproduce 1. Have a project with default branch `master` that has commits 2. Create a merge request via API with `source_branch: feature/my-feature` and `target_branch: main` (where `main` doesn't exist) 3. The MR is created successfully but shows: - `"diff_refs": null` - `"has_conflicts": true` - `"changes_count": null` - No diffs visible in UI - No clear indication that the target branch doesn't exist ## Expected Behavior - The API should validate that the target branch exists and return a clear error (400 Bad Request) if it doesn't - If the MR is somehow created, the UI should clearly indicate "Target branch 'main' does not exist" - GitLab Duo Chat/Agents should be aware of the project's default branch and prevent targeting non-existent branches ## Actual Behavior - MR is created successfully despite targeting a non-existent branch - No clear error message about missing target branch - User must manually discover the issue and change target branch - The MR appears broken with no explanation ## Workaround Update the MR to target an existing branch (e.g., `master`) ## Impact - Confusing for GitLab Duo Chat users who may not know the project's branch structure - Wastes time debugging what appears to be a GitLab bug - Poor API validation allows invalid state ## Environment - GitLab Version: 18.10.0 (GitLab.com SaaS) - API Version: v4 ## Additional Context Issue text was generated by GitLab Duo Chat (Claude Sonnet 4.5 - Anthropic model)