Fix approval rules webhook payload showing ActiveRecord proxy

What does this MR do and why?

Fix approval rules webhook payload showing ActiveRecord proxy

When updating a merge request without changing approval rules, the webhook payload incorrectly serializes the approval_rules.previous value as an ActiveRecord association proxy object string instead of properly converting it to an array.

References

Resolves #577568

How to set up and validate locally

1. Setup a merge request webhook and then change reviewers using the assign option, see:

"changes": {
    "approval_rules": {
      "previous": "#<ApprovalMergeRequestRule::ActiveRecord_Associations_CollectionProxy:0x0000000379856f48>",
      "current": [
        {
          "id": 1,
          "approvals_required": 0,
          "name": "All Members",
          "rule_type": "any_approver",
          "report_type": null,
          "merge_request_id": 37,
          "section": null,
          "modified_from_project_rule": false,
          "orchestration_policy_idx": null,
          "vulnerabilities_allowed": 0,
          "scanners": [],
          "severity_levels": [],
          "vulnerability_states": [
            "new_needs_triage",
            "new_dismissed"
          ],
          "security_orchestration_policy_configuration_id": null,
          "scan_result_policy_id": null,
          "applicable_post_merge": null,
          "project_id": 2,
          "approval_policy_rule_id": null,
          "updated_at": "2025-10-16 18:41:57 UTC",
          "created_at": "2025-10-16 18:41:57 UTC"
        }
      ]
    },

2. Checkout this branch and change reviewers in the same way and see:

"changes": {
    "approval_rules": {
      "previous": [],
      "current": [
        {
          "id": 1,
          "approvals_required": 0,
          "name": "All Members",
          "rule_type": "any_approver",
          "report_type": null,
          "merge_request_id": 37,
          "section": null,
          "modified_from_project_rule": false,
          "orchestration_policy_idx": null,
          "vulnerabilities_allowed": 0,
          "scanners": [],
          "severity_levels": [],
          "vulnerability_states": [
            "new_needs_triage",
            "new_dismissed"
          ],
          "security_orchestration_policy_configuration_id": null,
          "scan_result_policy_id": null,
          "applicable_post_merge": null,
          "project_id": 2,
          "approval_policy_rule_id": null,
          "updated_at": "2025-10-16 18:41:57 UTC",
          "created_at": "2025-10-16 18:41:57 UTC"
        }
      ]
    },

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