Extracts Resolve Vulnerability completion helpers

What does this MR do and why?

This MR is a simplified version of Add AIGW service class for resolve_vulnerability (!165884).

The purpose of this change is to extract shared code so that it can be used by the current completion (Gitlab::Llm::Completions::ResolveVulnerability) and the new implementation (Gitlab::Llm::AiGateway::Completions::ResolveVulnerability) which will be added in a follow up MR.

There are no functional changes in this MR, and the existing completion is tested in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/lib/gitlab/llm/completions/resolve_vulnerability_spec.rb

All methods in the helper file are private, so no tests were moved as part of the extraction, and no new tests were added.

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