Backport of 'Fail early when an API-only flow's image has no curl'

What does this MR do and why?

Backport of !264009 (merged) to 19.5.

Flows with coding_environment: none (for example Recommend Reviewers) skip the project's setup_script but still run in the image from .gitlab/duo/agent-config.yml. When that image relies on setup_script for curl, the duo-cli download fails with curl: command not found, which doesn't tell the customer what to fix. This keeps the configured image and checks for curl first. If it's missing, the job stops with a message that says the flow skips setup_script and to add curl or duo to the image. Which image runs is unchanged, so it's safe for locked-down self-managed runners.

Details

• One conflict on start_workflow_service.rb: master's installed_duo_check_command (GitLab-hosted runners) isn't on 19.5, so only curl_check_command is added. The rest applied cleanly.
• start_workflow_service_spec.rb and update_workflow_status_service_spec.rb pass on the 19.5 base (266 examples).

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 Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), 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.

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).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.

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