Fix Amazon Q usage quota check

What does this MR do and why?

Fixes the Amazon Q MR review feature and stops a Unavailable - no credits error message appearing when the Amazon Q service user is visible.

Amazon Q is licensed via a Duo with Amazon Q subscription, not via Duo Agent Platform usage credits, so its service account must not be subjected to a DAP usage quota check even though it is a composite identity agent.

References

• Customer RFH: https://gitlab.com/gitlab-com/request-for-help/-/work_items/4859
• Likely cause of the "Unavailable - no credits" error !228141 (merged)

Note: The customer RFH refers to assigning Amazon Q as a reviewer. That does not work and is not intended to. The feature works via a /q review slash command in a comment in an MR, as documented. Amazon Q can also review code in merge requests automatically, if enabled in admin Amazon Q settings.

How to set up and validate locally

1. Follow these instructions to set up a test (non-GDK) instance in an AWS sandbox: https://docs.gitlab.com/development/ai_features/amazon_q_integration/
   • Note the instructions are out of date regarding the license. It says you can just use an Ultimate subscription. That's no longer correct, it needs to be a GitLab Duo with Amazon Q subscription - Not an add-on but a specific subscription type. I can share mine, or someone in #s_fulfillment_engineering can create one for you.
2. Create an MR and enter /q review as a comment

MR acceptance checklist

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