…ated lastRun.outcomeMsg (elastic#258105)

**Resolves: elastic#177852
**Relates to: elastic#251164

## Summary

This PR fixes an issue blocking enabling and disabling security rules with non-migrated `lastRun.outcomeMsg` field.

## Details

Some time ago Alerting Framework migrated `lastRun.outcomeMsg` from `string` to `string[]`. At that moment usual migrations were deprecated due to Serverless and model version migration wasn't ready yet. This migration went smoothly thanks to the according changes to the codebase. However, the changes didn't cover customers upgrading from `7.x` stack version and having something written to `lastRun.outcomeMsg`.

elastic#251164 fixed the issue appearing in attempt to update prebuilt rules.

This PR fixes the left issue blocking enabling and disabling non-migrated security rules with `lastRun.outcomeMsg` type `string` field.

## Testing

- Start Kibana
- Log in under `system_indices_superuser` to be able to write to the system indices
- Create a rule by using the command below
- Enable/Disable the rule
- Perform any other bulk and non-bulk actions on the rule

ER: All actions should work without errors.

Without this fix enable/disable action will result in error.

<details>
  <summary>ES command to put a non-migrated rule</summary>

```
PUT .kibana_alerting_cases/_doc/alert:d62167ce-1022-4b2c-915d-024fe3e6e557
{
    "alert": {
      "name": "Test rule 1",
      "tags": [],
      "enabled": false,
      "alertTypeId": "siem.queryRule",
      "consumer": "siem",
      "legacyId": null,
      "schedule": {
        "interval": "5m"
      },
      "actions": [],
      "params": {
        "author": [],
        "description": "123",
        "falsePositives": [],
        "from": "now-6m",
        "ruleId": "5ecfb16d-5af2-4a31-b880-319a4a2ca92b",
        "immutable": false,
        "ruleSource": {
          "type": "internal"
        },
        "license": "",
        "outputIndex": "",
        "meta": {
          "kibana_siem_app_url": "http://localhost:5601/kbn/app/security"
        },
        "maxSignals": 100,
        "riskScore": 21,
        "riskScoreMapping": [],
        "severity": "low",
        "severityMapping": [],
        "threat": [],
        "to": "now",
        "references": [],
        "version": 1,
        "exceptionsList": [],
        "relatedIntegrations": [],
        "requiredFields": [],
        "setup": "",
        "type": "query",
        "language": "kuery",
        "index": [
          "apm-*-transaction*",
          "auditbeat-*",
          "endgame-*",
          "filebeat-*",
          "logs-*",
          "packetbeat-*",
          "traces-apm*",
          "winlogbeat-*",
          "-*elastic-cloud-logs-*"
        ],
        "query": "*:*",
        "filters": []
      },
      "mapped_params": {
        "risk_score": 21,
        "severity": "20-low"
      },
      "createdBy": "elastic",
      "updatedBy": "elastic",
      "createdAt": "2026-03-17T10:52:28.688Z",
      "updatedAt": "2026-03-17T10:52:28.688Z",
      "apiKey": null,
      "apiKeyOwner": null,
      "apiKeyCreatedByUser": null,
      "throttle": null,
      "notifyWhen": null,
      "muteAll": false,
      "mutedInstanceIds": [],
      "executionStatus": {
        "status": "pending",
        "lastExecutionDate": "2026-03-17T10:52:28.688Z"
      },
      "monitoring": {
        "run": {
          "history": [],
          "calculated_metrics": {
            "success_ratio": 0
          },
          "last_run": {
            "timestamp": "2026-03-17T10:52:28.688Z",
            "metrics": {
              "duration": 0,
              "total_search_duration_ms": null,
              "total_indexing_duration_ms": null,
              "total_alerts_detected": null,
              "total_alerts_created": null,
              "gap_duration_s": null
            }
          }
        }
      },
      "lastRun": {
        "outcome": "failed",
        "outcomeMsg": "security_exception: [security_exception] Reason: missing authentication credentials for REST request [/_security/user/_has_privileges], caused by: \"\"",
        "warning": "read",
        "alertsCount": {},
        "outcomeOrder": 20
    },
      "snoozeSchedule": [],
      "revision": 0,
      "running": false,
      "artifacts": {
        "dashboards": [],
        "investigation_guide": {
          "blob": ""
        }
      },
      "meta": {
        "versionApiKeyLastmodified": "9.4.0"
      }
    },
    "type": "alert",
    "references": [],
    "managed": false,
    "namespaces": [
      "default"
    ],
    "coreMigrationVersion": "8.8.0",
    "typeMigrationVersion": "10.10.0",
    "updated_at": "2026-03-17T10:52:28.688Z",
    "created_at": "2026-03-17T10:52:28.688Z"
  }
```
</details>

(cherry picked from commit b48ac09)

# Conflicts:
#	x-pack/platform/plugins/shared/alerting/server/application/rule/methods/bulk_enable/bulk_enable_rules.test.ts