Draft: [Spike] MR Widget - Backend MVC for MR Approval Policies Warn Mode

What does this MR do and why?

Introduces enforcement_type for scan result policies, allowing them to be configured as 'warn' or 'enforce'. 'Warn' mode policies will not block merge requests if their violations are dismissed, providing more flexibility in security policy enforcement.

This includes database schema updates, model changes, and mergeability service adjustments to support the new behavior.

References

Screenshots or screen recordings

Monosnap_screencast_2025-08-22_5_PM-59-37

Before	After

How to set up and validate locally

1. Create security policy for a project

---
approval_policy:
- name: MR - Security Scan
  description: Security Scan
  enabled: true
  enforcement_type: "warn"
  rules:

  - type: scan_finding
    scanners:
    - sast
    vulnerabilities_allowed: 0
    severity_levels: []
    vulnerability_states: []
    branch_type: protected

  actions:
  - type: require_approval
    approvals_required: 1
    role_approvers:
    - developer
    - maintainer
    - owner
  - type: send_bot_message
    enabled: true
  approval_settings:
    block_branch_modification: false
    prevent_pushing_and_force_pushing: false
    prevent_approval_by_author: false
    prevent_approval_by_commit_author: false
    remove_approvals_with_new_commit: false
    require_password_to_approve: false
  fallback_behavior:
    fail: open

2. Create MR with security policy violations - MR is blocked from security policy violations
3. Dismiss the violations

mr = MergeRequest.find(<id>)
project = Project.find_by_full_path(<path>)
Security::SecurityOrchestrationPolicies::DismissForMergeRequestService.new(mr, proj
ect.security_policies.first, User.find_by_username('root') ).execute

4. MR is unblocked and allowed to merged
5. Make additional commits in the MR, the dismissal is persisted
6. Introduce new violations for the same policy - MR is blocked again
7. [For testing purpose] If we delete the dismissal records, MR is blocked again

Security::PolicyDismissal.where(merge_request: @mr).delete_all

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.

Related to #536153 (closed)