ESQL: columns lost with simple INLINESTATS query. EsqlSession.fieldNames test coverage needed

### Description

```
FROM employees | INLINESTATS max_salary = MAX(salary) by languages
```

results in (only columns provided below)

```
languages.byte |languages.long |languages.short|    salary     |  max_salary   |   languages   
```

which is missing quite a big bunch of the initial columns. This shows a big lack of support (and testing coverage) in `EsqlSession.fieldNames` method.