Provide descriptive error when Duo Code Review fails due to missing gitlab--duo runner tag
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## Description
Customers enabling **Duo Code Review** after purchasing **DAP credits** are encountering generic pipeline timeouts that lead to significant frustration.
The root cause is often that the project lacks a runner with the `gitlab--duo` tag. Per our documentation, from **GitLab 18.7 onward**, AI workloads strictly use runners containing this specific tag. Without it, the GitLab code review flow cannot find a suitable runner and eventually times out without a descriptive error pointing to the missing tag.
### Current Behavior
* Pipeline for Code Review times out.
* Error message is generic and does not mention runner tags.
* Customer is left confused despite having credits and features enabled.
### Proposed Improvement
We should update the error message or provide a pre-flight check in the UI/Pipeline logs that explicitly identifies the missing `gitlab--duo` tag.
> **Recommended Pattern (Documentation Ref):** Use both `gitlab--duo` and "run untagged jobs" until 18.7, then disable untagged once fully migrated to the tag.
## Definition of Done
* \[ \] Implement a specific error message when a Duo Code Review job fails to find a runner with the `gitlab--duo` tag.
* \[ \] (Optional) Add a warning in the Duo configuration UI if no runners with the `gitlab--duo` tag are detected for the instance/project.
issue