Create AI flow pipeline on mention

What does this MR do and why?

• Extracted from Draft: AI Flow triggers (!200193 - closed)
• Related to https://gitlab.com/gitlab-org/gitlab/-/issues/559051+

This MR implements the AI flow pipeline trigger functionality that allows users to trigger AI workflows when mentioned in comments. This feature enables automated AI-powered responses and actions based on user mentions in GitLab issues and merge requests.

Testing

Ensure you have enabled ai_flow_triggers feature flag.

Similar to Draft: AI Flow triggers (!200193 - closed) but Ai flow trigger should be created via Rails console:

::Ai::FlowTrigger.create!(project: project, user: user, event_types: [0], description: "Run AI Flow Trigger", config_path: ".gitlab/duo/agents.yml")