Add support for Codestral-on-Vertex for Rails-AIGW requests

We added support for Codestral-on-Vertex on the AI Gateway: Add Codestral on Vertex support for code comple... (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#564 - closed). We also added a feature flag (use_codestral_for_code_completions) that would toggle the use of Codestral-on-Vertex for direct connections requests (from the IDE to AI Gateway): Add Codestral-on-Vertex model details to /code_... (#479939 - closed).

We need to make sure that the Rails -> AIGW code completion requests specifies Codestral-on-Vertex when use_codestral_for_code_completions is enabled.