Quarantine flaky agentic chat custom-agent spec [19.1]

What does this MR do and why?

Quarantine the allows user to select a custom agent example on this stable branch.

A recent change on the AI gateway main branch broke the custom-agent mock path, so the chat renders one .duo-chat-message instead of two and the example times out after 30 seconds. The rspec-ee system pg17 duo_workflow_service job clones the AI gateway from its moving main branch (DEFAULT_BRANCH in ee/lib/tasks/gitlab/ai_gateway/utils.rb), so this fails consistently on stable branches.

On the default branch the whole example was removed by !244590 (merged), so master is not affected. This MR is the stable-branch equivalent: it quarantines the one example to unblock CI while the durable fix lands on the AI gateway side.

This change adds only quarantine metadata and modifies no product code.

Related issue: #628000

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).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

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