Adding readiness score argument to link vulnerability merge request mutation

What does this MR do and why?

This MR adds a readiness score argument to the VulnerabilityLinkMergeRequest mutation, this will represent the confidence of the ai workflow created resolution MR.

References

#570181

How to set up and validate locally

1. Navigate to a project with a vulnerability
2. Create an MR
3. In the graphql explorer /-/graphql-explorer make the following mutation

mutation linkMergeRequest {
  vulnerabilityLinkMergeRequest(input: {
    vulnerabilityId: "gid://gitlab/Vulnerability/<id>",
    mergeRequestId: "gid://gitlab/MergeRequest/<id>",
    readinessScore: 0.3
  }) {
    vulnerability {
      id
      mergeRequests {
        nodes {
          id
          title
        }
      }
    }
    errors
  }
}

4. You can check the merge request link record in rails console and whether the readinessScore was updated or not by running following command, ensure readiness score is there:

Vulnerabilities::MergeRequestLink.last

MR acceptance checklist

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

Related to #570181