Add ability to modify service name when using chat

What does this MR do and why?

When using new slash commands, we need to specify the unit primitive this slash command requires.

We need to check it in Web UI, but also we need to pass it to AI Gateway for second verification.

This is how @nmilojevic1 described it: "So we can send something like 'X-Gitlab-Unit-Primitive' => root_cause_analysis, as a request header and verify in /chat endpoint if the token contains root_cause_analysis scope"

This MR adds this mechanism to chat codebase. To leverage it, we need to override context.ai_request in the slash command class with ::Gitlab::Llm::Chain::Requests::AiGateway.new(user, service_name: :something, tracking_context: tracking_context).

We need AI gateway work as well to make it work: we need in /chat endpoint something similar as in proxy endpoints(https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/ai_gateway/api/v1/proxy/anthropic.py#L29-31)

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related to #467382