Vendored AI features: Fix params for vendored features

What does this MR do and why?

For #525918 (closed).

When an AI feature is set to vendored, the params being sent to the cloud AIGW should mock params as if the gitlab-default model for that feature is being requested. Essentially, we are re-using the params we designed for the model-switching framework and repurposing it here for use in self-hosted models, when the feature is vendored.

With this change, all of chat features, non-chat features and code generation should use these params when in vendored mode. Code completions alone does not use these params, and that will be fixed in a follow-up MR.

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