Self-managed support for Git Natural Language Assistant

Overview

We originally built this AI feature independently of the AI Gateway (AI GW). In order to mature the feature to GA, we need it to support self-managed instances.

This is Phase 1A of using the AI GW to access LLMs. More context: AI Gateway as the Sole Access Point for Monolit... (&13024)

Proposal

We need to determine if the monolith part of this feature can just use the existing Gitlab::Llm::Vertex::Client client that groupai framework is working on refactoring during Phase 1A (&13397 (closed)), which should enable the feature to be routed via the AI GW.

On the CLI side, we must make sure the ask command respects which hosts is configured (SaaS or self-managed).

Once &13397 (closed) is complete, we should test and verify that the feature still works as expected.