The ES|QL `COUNT_DISTINCT` function fails with an `illegal agg type: text` error when it's run on a `text` field. It's fine with `keyword` fields. Compare to for example `COUNT`, which runs fine on text fields:

```
PUT test/_doc/1
{
  "foo": "bar"
}

# This fails with illegal agg type: text:
POST /_query
{
  "query": """
    FROM test
    | STATS count = COUNT_DISTINCT(foo)
  """
}

# This works:
POST /_query
{
  "query": """
    FROM test
    | STATS count = COUNT_DISTINCT(foo.keyword)
  """
}

# COUNT is fine with text fields:
POST /_query
{
  "query": """
    FROM test
    | STATS count = COUNT(foo)
  """
}
```

Full error message:

```
{
  "error": {
    "root_cause": [
      {
        "type": "esql_illegal_argument_exception",
        "reason": "illegal agg type: text"
      }
    ],
    "type": "esql_illegal_argument_exception",
    "reason": "illegal agg type: text"
  },
  "status": 500
}
```
x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/EsqlActionIT.java
builder.field("type", "nested");
                    builder.startObject("properties");
                    {
                        builder.startObject("foo");
                        builder.field("type", "long");
                        builder.endObject();
                    }
                    builder.endObject();
                }
                builder.endObject();
                builder.startObject("data");
                builder.field("type", "long");
                builder.endObject();
            }
            builder.endObject();
        }
        builder.endObject();
        assertAcked(
            client().admin()
                .indices()
                .prepareCreate(indexName)
                .setSettings(Settings.builder().put("index.number_of_shards", ESTestCase.randomIntBetween(1, 3)))
                .setMapping(builder)
        );
    }
    private int indexDocsIntoNestedMappingIndex(String indexName, int docsCount) throws IOException {
        int countValuesGreaterThanFifty = 0;
        BulkRequestBuilder bulkBuilder = client().prepareBulk();
        for (int j = 0; j < docsCount; j++) {
            XContentBuilder builder = JsonXContent.contentBuilder();