Follow-up from "Add GitLab Duo runbook": AI vendor limits

The following discussion from !7792 (merged) should be addressed:

• @mkaeppler started a discussion: (+5 comments)

  @oregand @shinya.maeda Sorry, I feel like I already asked this question but lost the link: do you know how these errors would surface through the AIGW and to the end-user, e.g. when VertexAI cuts us off because our vendor limits are reached?

TODO: document failure modes and solutions for exhausting AI vendor quotas