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
static final List<String> NUMERIC = List.of("Int", "Long", "Double");
    
    static final List<? extends Class<? extends Function>> AGG_FUNCTIONS = List.of(
        Count.class,
        CountDistinct.class,
        Max.class,
        Median.class,
        MedianAbsoluteDeviation.class,
        Min.class,
        Percentile.class,
        Sum.class
    );
    
    record AggDef(Class<?> aggClazz, String type, boolean grouping) {}
    
    private final Map<AggDef, List<IntermediateStateDesc>> mapper;
    
    private final HashMap<Expression, List<? extends NamedExpression>> cache = new HashMap<>();