19.3 Backport of 'Remove merge_request from Developer default triggers'

What does this MR do and why?

Backports commit c1dd9ab3bd19 ("Remove merge_request from Developer default triggers") from !250220 (merged) to 19-3-stable-ee, for #628094.

On 19.3, enabling the Developer foundational flow auto-creates a generic merge_request flow trigger with no action filter, so flow runs start on merge request events even when the user never opted in to a merged-MR trigger. This backport removes ::Ai::FlowTrigger::EVENT_TYPES[:merge_request] from the Developer flow's default triggers (leaving assign and mention) and updates the two related specs:

• ee/app/models/ai/catalog/foundational_flow/definitions/developer.rb
• ee/spec/services/ai/catalog/flows/sync_foundational_flows_service_spec.rb
• ee/spec/services/ai/catalog/item_consumers/create_service_spec.rb

The commit cherry-picked cleanly; the resulting diff is byte-identical to the original commit on the default branch. Only the trigger-removal commit is backported — the rest of MR 250220 (the MergeRequests::MergedCloudEvent rework) is 19.4 scope and intentionally excluded. SyncFoundationalFlowsService is create-only, so previously synced merge_request trigger rows are not deleted by this change.

References

• Original MR: !250220 (merged)
• Original commit: c1dd9ab3
• Issue: #628094

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.

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.