### 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
int lineNumber = 1;
            while ((line = reader.readLine()) != null) {
                line = line.trim();
                
                if (shouldSkipLine(line) == false) {
                    String[] entries = multiValuesAwareCsvToStringArray(line, lineNumber);
                    
                    if (columns == null) {
                        columns = new CsvColumn[entries.length];
                        for (int i = 0; i < entries.length; i++) {
                            int split = entries[i].indexOf(":");
                            String name, typeName;
                            if (split < 0) {
                                throw new IllegalArgumentException(
                                    "A type is always expected in the schema definition; found " + entries[i]
                                );
                            } else {
                                name = entries[i].substring(0, split).trim();
                                typeName = entries[i].substring(split + 1).trim();
                                if (typeName.length() == 0) {
                                    throw new IllegalArgumentException(
                                        "A type is always expected in the schema definition; found " + entries[i]
                                    );
                                }
                            }
                            Type type = Type.asType(typeName);
                            if (type == null) {
                                throw new IllegalArgumentException("Can't find type for " + entries[i]);
                            }
                            if (type == Type.NULL) {