Ensure auto_merge_enabled is set when validating merge trains

What does this MR do and why?

Backport of !170652 (merged) to %17.4

Merge trains can get stuck if there is inconsistent state between the merge request and merge train.

This fix allows merge trains to recover from such inconsistent state the next time the train is refreshed. This, happens, for example, when a pipeline completes, and when a merge request is added or removed from the train. Corrected MRs will show two cancellation-related system notes: One for the original cancellation action that left the MR in an inconsistent state, and one for the merge train cleanup caused by the new validation being enforced.

This MR is being backported because the it is part of a high-value customer escalation and the customer is several releases behind. This 17.4 backport is the goal, but there is also a 17.5 backport for parity .

See #500887

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).