### Elasticsearch Version

8.8.2

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Mac OS X

### Problem Description

Under certain conditions, fetching a particular field value using the "fields" API can lead to `string_index_out_of_bounds_exception`.
The queried field needs to be inside a "nested" object and in addition to the field name, all fields need to be queries as well (`*`).
Both fields requested also need to be requested with `"include_unmapped": true`. 

### Steps to Reproduce

```
DELETE testindex

PUT testindex
{
    "mappings": {
        "dynamic": "false",
        "properties": {
            "nested_field": {
                "type": "nested",
                "properties": {
                    "file": {
                        "type": "keyword"
                    }
                }
            }
        }
    }
}

POST testindex/_doc
{
    "nested_field": {        
            "file": "somefile.txt"
    }
}

GET testindex/_search
{
  "_source": false,
  "fields": [
    {
      "field": "file",
      "include_unmapped": true
    },
    {
      "field": "*",
      "include_unmapped": true
    }
  ],
  "query": {
    "match_all" : {}
  }
}
```

This gives the following error:
```
{
    "error": {
        [...],
        "caused_by": {
            "type": "string_index_out_of_bounds_exception",
            "reason": "Range [13, 4) out of bounds for length 4",
            "caused_by": {
                "type": "string_index_out_of_bounds_exception",
                "reason": "Range [13, 4) out of bounds for length 4"
            }
        }
    },
    "status": 500
}
```

### Logs (if relevant)

_No response_
server/src/main/java/org/elasticsearch/search/fetch/subphase/UnmappedFieldFetcher.java
if (this.unmappedFieldsFetchAutomaton.isAccept(currentState)) {
                        if (value != null) {
                            DocumentField currentEntry = documentFields.get(currentPath);
                            if (currentEntry == null) {
                                List<Object> list = new ArrayList<>();
                                list.add(value);
                                documentFields.put(currentPath, new DocumentField(currentPath, list));
                            } else {
                                currentEntry.getValues().add(value);
                            }
                        }
                    }
                }
            }
        }
        
        if (this.unmappedConcreteFields != null) {
            for (String path : unmappedConcreteFields) {
                if (this.mappedFields.contains(path)) {
                    continue; 
                }
                List<Object> values = XContentMapValues.extractRawValues(path, source);
                if (values.isEmpty() == false) {
                    documentFields.put(path, new DocumentField(path, values));
                }
            }
        }
    }
    private void collectUnmappedList(Map<String, DocumentField> documentFields, Iterable<?> iterable, String parentPath, int lastState) {