Add _data_stream metadata to SearchHit for _search response

[seanzatzdev]

Per #112750 , this PR adds the _data_stream metadata field to the Search Hit class and displays it in the response for _search

e.g.

{
    "took": 47,
    "timed_out": false,
    "_shards": {
        "total": 1,
        "successful": 1,
        "skipped": 0,
        "failed": 0
    },
    "hits": {
        "total": {
            "value": 1,
            "relation": "eq"
        },
        "max_score": 1.0,
        "hits": [
            {
                "_index": ".ds-logs-myapp-default-2025.08.05-000001",
                "_data_stream": "logs-myapp-default",
                "_id": "AZh64tcIiR68mRpOTMUC",
                "_score": 1.0,
                "_source": {
                    "@timestamp": "2099-05-06T16:21:15.000Z",
                    "message": "192.0.2.42 -[06/May/2099:16:21:15] \"GET /images/bg.jp..."
                }
            }
        ]
    }
}

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[elasticsearchmachine]

Hi @seanzatzdev, I've created a changelog YAML for you.

[masseyke]

I think we decided not to do this one because we don't want to introduce the runtime overhead of determining if the index is actually in a data stream to this API.