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/main/java/org/elasticsearch/xpack/esql/planner/AggregateMapper.java
import org.elasticsearch.xpack.esql.expression.function.aggregate.CountDistinct;
import org.elasticsearch.xpack.esql.expression.function.aggregate.Max;
import org.elasticsearch.xpack.esql.expression.function.aggregate.Median;
import org.elasticsearch.xpack.esql.expression.function.aggregate.MedianAbsoluteDeviation;
import org.elasticsearch.xpack.esql.expression.function.aggregate.Min;
import org.elasticsearch.xpack.esql.expression.function.aggregate.NumericAggregate;
import org.elasticsearch.xpack.esql.expression.function.aggregate.Percentile;
import org.elasticsearch.xpack.esql.expression.function.aggregate.Sum;
import org.elasticsearch.xpack.ql.expression.Alias;
import org.elasticsearch.xpack.ql.expression.AttributeMap;
import org.elasticsearch.xpack.ql.expression.Expression;
import org.elasticsearch.xpack.ql.expression.FieldAttribute;
import org.elasticsearch.xpack.ql.expression.MetadataAttribute;
import org.elasticsearch.xpack.ql.expression.NamedExpression;
import org.elasticsearch.xpack.ql.expression.ReferenceAttribute;
import org.elasticsearch.xpack.ql.expression.function.Function;
import org.elasticsearch.xpack.ql.expression.function.aggregate.AggregateFunction;
import org.elasticsearch.xpack.ql.tree.Source;
import org.elasticsearch.xpack.ql.type.DataType;
import org.elasticsearch.xpack.ql.type.DataTypes;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class AggregateMapper {