feat(telemetry): log 'X-Gitlab-Duo-Seat-Count' header value

What does this merge request do and why?

We discovered that we missed logging X-Gitlab-Duo-Seat-Count. This MR fixes that.

Resolves https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/606

How to set up and validate locally

The simplest way to verify is to open http://localhost:3000/admin/code_suggestions (assuming you have working GDK <-> local AI GW setup). This page will send a test Code Completion request:

You can see gitlab_duo_seat_count in request logs:

The value will match the header.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Closes #606