Add integration test for policy dismissal audit events

What does this MR do and why?

This MR adds a comprehensive integration test for the policy dismissal audit event functionality that was implemented in !205857 (merged).

The test verifies the complete flow:

1. Creating a security policy in warn mode
2. Creating a merge request with policy violations
3. Dismissing the policy violations
4. Merging the MR
5. Verifying that audit events are created correctly

Test Coverage

The integration test covers:

• Happy path: When a merge request with dismissed policy violations is merged, an audit event is created
• Edge case: When policy dismissals are not applicable for all violations, they are destroyed and no audit event is created
• Negative case: When there are no policy dismissals, no audit events are created

Key Test Scenarios

1. Audit Event Creation

• Verifies that when an MR with dismissed policies is merged, the policy dismissal status changes from open to preserved
• Confirms that an audit event is created with the correct:
  • Author (the user who dismissed the policy)
  • Target (the security policy)
  • Message (includes MR reference)
  • Entity (the project)

2. Non-Applicable Dismissals

• Tests the scenario where a dismissal doesn't cover all violation UUIDs
• Verifies that such dismissals are destroyed during the merge process
• Confirms no audit event is created for destroyed dismissals

3. No Dismissals

• Ensures that MRs without policy dismissals don't trigger audit events

Implementation Details

The test uses:

• Real security policy configuration setup
• Actual merge request creation and merging
• Background job processing (perform_enqueued_jobs)
• Factory-created policy dismissals and violations
• Proper feature flag and license stubbing

References

• Related to: !205857 (merged) (original implementation)
• Follows the validation steps from the original MR description
• Tests the functionality described in #569628

How to set up and validate locally

1. Enable the feature flag: Feature.enable(:security_policy_approval_warn_mode)
2. Run the test: bundle exec rspec ee/spec/features/merge_requests/policy_dismissal_audit_integration_spec.rb

The test should pass and verify all the audit event creation scenarios.

MR acceptance checklist

• Test covers the main functionality
• Test covers edge cases
• Test uses proper factories and setup
• Test includes proper assertions
• Test follows GitLab testing conventions