Add extended completions

What does this MR do and why?

Extend Gitlab::Llm::CompletionsFactory for JH.

This MR extracts common logic for MRs:

• !1810 (merged)
• !1798 (merged)

::Gitlab::Llm::ClientFactory.ai_provider is the option entry of AI provider, through ::Settings.gitlab['ai_provider'].

Gitlab::Llm::CompletionsFactory allows each specific provider (such as ChatGLM or other AI provider) to provide alternative implementations.

Screenshots or screen recordings

Non UI

How to set up and validate locally

MR acceptance checklist

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