Quarantine Custom model features specs

What does this MR do and why?

Quarantine test in 17-5 branch which is incompatible with ai-gateway. It has been quarantined in 17.6

It's causing failures: References: https://gitlab.com/gitlab-org/security/gitlab/-/jobs/8590842301 https://gitlab.com/gitlab-org/gitlab/-/jobs/8590825548

See thread: https://gitlab.slack.com/archives/C3JJET4Q6/p1733775980900019

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).