### Elasticsearch Version

main

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

all

### Problem Description

Do this

### Steps to Reproduce

```
rm /tmp/evil
for a in {0..9}; do
   for b in {0..9}; do
      echo '{"index": {}}' >> /tmp/evil
      echo '{"a": '$a', "b": '$b'}' >> /tmp/evil
   done
done
echo >> /tmp/evil

curl -uelastic:password -XDELETE localhost:9200/test
curl -HContent-Type:application/json -uelastic:password -XPUT localhost:9200/test
curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/test/_bulk --data-binary @/tmp/evil
curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/test/_forcemerge?max_num_segments=1
curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/test/_refresh

curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/_esql?pretty -d'{
   "query": "FROM test | STATS MAX(a) BY a, b | KEEP a"
}'
```

### Logs (if relevant)

_No response_
x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestUtils.java
format(
                                "Error line [{}:{}]: Unexpected missing value in a multi-value column; found [{}]",
                                lineNumber,
                                pos,
                                csvLine.substring(pos - 1)
                            )
                        );
                    }
                    mvValue.append("," + entry);
                } else {
                    mvCompressedEntries.add(entry);
                }
            }
            pos = 1 + (isLastElement ? csvLine.length() : commaPos);
        }
        return mvCompressedEntries.toArray(String[]::new);
    }
    public record ExpectedResults(List<String> columnNames, List<Type> columnTypes, List<List<Object>> values) {}
    public static ExpectedResults loadCsvSpecValues(String csv) {
        List<String> columnNames;
        List<Type> columnTypes;
        try (CsvListReader listReader = new CsvListReader(new StringReader(csv), CSV_SPEC_PREFERENCES)) {
            String[] header = listReader.getHeader(true);
            columnNames = new ArrayList<>(header.length);
            columnTypes = new ArrayList<>(header.length);
            for (String c : header) {
                String[] nameWithType = Strings.split(c, ":");
                if (nameWithType == null || nameWithType.length != 2) {
                    throw new IllegalArgumentException("Invalid CSV header " + c);
                }