### 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
if (entry.endsWith("]")) {
                    if (entry.length() > 2) {
                        mvCompressedEntries.add(entry.substring(1, entry.length() - 1));
                    } else {
                        mvCompressedEntries.add("");
                    }
                } else {
                    mvValue = new StringBuilder();
                    previousMvValue = entry.substring(1);
                    mvValue.append(previousMvValue);
                }
            } else if (entry.endsWith("]")) {
                if (previousMvValue == null) {
                    throw new IllegalArgumentException(
                        format(
                            "Error line [{}:{}]: Unexpected end of a multi-value field value (no previous starting point); found [{}]",
                            lineNumber,
                            pos,
                            entry
                        )
                    );
                }
                mvValue.append("," + entry.substring(0, entry.length() - 1));
                mvCompressedEntries.add(mvValue.toString());
                mvValue = null;
                previousMvValue = null;
            } else {
                if (mvValue != null) {
                    if (entry.length() == 0) {
                        throw new IllegalArgumentException(