Allow GitLab Duo Chat QA Evaluation (rspec ver.) to request to AI Gateway

Problem

Currently, the rspec test of GitLab Duo Chat QA Evaluation is requesting to Anthropic directly, however, the request should go to AI Gateway instead to align with the actual production environment.

Proposal

Support AI Gateway in rspec test jobs.

Related !138274 (merged)

Edited by Shinya Maeda