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 - closed).

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.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

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