Add skip_cache option

What does this MR do and why?

Add skip_cache option

It's possible that some AI chain tools use completion service (e.g. SummaryComments tool), in this case we want to avoid storing request/response in cache because it's only intermediate step.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ask a question which uses SummarizeComments tool (e.g. "what is summary of comments on flightjs/Flight#1 issue?")
2. Or alternatively can be verified from console with something like: res = Gitlab::Llm::Chain::Agents::ZeroShot::Executor.new(user_input: 'What is summary of comments on flightjs/Flight#1 issue?', tools: [Gitlab::Llm::Chain::Tools::IssueIdentifier,Gitlab::Llm::Chain::Tools::SummarizeComments], context: ::Gitlab::Llm::Chain::GitlabContext.new(current_user: user,container:nil,resource: <issue>, ai_request: ::Gitlab::Llm::Chain::Requests::Anthropic.new(user))).execute
3. Verify in rails console that no "intermediate" messages are stored in cache using Gitlab::Llm::Cache.new(user).find_all

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.