Implement method to sync approved approvers after merge

Captures which users approved which rules for compliance.

Validate these requirements during implementation and adjust as needed:

• Add sync_approved_approvers method to MergeRequests::ApprovalRule model
• Only runs after MR is merged (check merge_request&.merged? and merge_request.finalizing_rules.present?)
• Uses ApprovalWrappedRule.wrap(merge_request, self).approved_approvers to get list of approvers
• Populates approved_approvers join table via approved_approver_ids=
• Used by existing ApprovalRules::FinalizeService (no service changes needed)

Reference: POC implementation in poc/v2-approval-rules-complete branch