### 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
collectUnmapped(documentFields, source.source(), "", 0);
    }
    private void collectUnmapped(Map<String, DocumentField> documentFields, Map<String, Object> source, String parentPath, int lastState) {
        
        if (this.unmappedFieldsFetchAutomaton != null) {
            for (String key : source.keySet()) {
                Object value = source.get(key);
                String currentPath = parentPath + key;
                if (this.mappedFields.contains(currentPath)) {
                    continue;
                }
                int currentState = step(this.unmappedFieldsFetchAutomaton, key, lastState);
                if (currentState == -1) {
                    
                    continue;
                }
                if (value instanceof Map) {
                    
                    @SuppressWarnings("unchecked")
                    Map<String, Object> objectMap = (Map<String, Object>) value;
                    collectUnmapped(
                        documentFields,
                        objectMap,
                        currentPath + ".",
                        step(this.unmappedFieldsFetchAutomaton, ".", currentState)
                    );
                } else if (value instanceof List) {
                    
                    collectUnmappedList(documentFields, (List<?>) value, currentPath, currentState);
                } else {