Bump duo-cli version from 9.8.0 to 9.19.0 in StartWorkflowService

What does this MR do and why?

Bumps the pinned @gitlab/duo-cli version in Ai::DuoWorkflows::StartWorkflowService from 9.8.0 to 9.19.0, the latest Duo CLI release.

Duo Developer flows are intermittently failing with WebSocket closed abnormally errors (see the example job linked below). Newer CLI versions include fixes for dropped WebSocket connections, so upgrading should resolve these failures — and 9.8.0 is quite outdated at this point, so it's worth upgrading regardless.

The version is pinned in a single place; CI (setup.gitlab-ci.yml) reads the constant dynamically to verify npm availability of the pinned version, and the spec references the constant directly, so no other files need changes.

References

• Example failure: WebSocket closed abnormally in job 16383009883
• Duo CLI v9.19.0 release
• Previous bump: !249280 (merged) (8.109.0 → 9.8.0)

Screenshots or screen recordings

N/A — version constant bump only.

Before	After
9.8.0	9.19.0

How to set up and validate locally

• Checkout this branch and trigger any flow (e.g. duo developer)
• Check that the flow executes end to end without any issues and that is uses the new CLI version (from the logs you should see which version gets installed)

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.