[BUG] Get conversation meta don't return application_type

[Hailong-am]

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current get memory api https://opensearch.org/docs/latest/ml-commons-plugin/api/memory-apis/get-memory/ will return response like below, it miss application_type field.

{
  "memory_id": "gW8Aa40BfUsSoeNTvOKI",
  "create_time": "2024-02-02T18:07:06.887061463Z",
  "updated_time": "2024-02-02T19:01:32.121444968Z",
  "name": "Conversation for a RAG pipeline",
  "user": "admin"
}

mapping of system index .plugins-ml-memory-meta

{
  ".plugins-ml-memory-meta": {
    "mappings": {
      "_meta": {
        "schema_version": 1
      },
      "properties": {
        "application_type": {
          "type": "keyword"
        },
        "create_time": {
          "type": "date",
          "format": "strict_date_time||epoch_millis"
        },
        "name": {
          "type": "text"
        },
        "updated_time": {
          "type": "date",
          "format": "strict_date_time||epoch_millis"
        },
        "user": {
          "type": "keyword"
        }
      }
    }
  }
}

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