### 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/test/java/org/elasticsearch/search/fetch/subphase/FieldFetcherTests.java
.endObject()
            .startObject("nullable_long_field")
            .field("type", "long")
            .field("null_value", 42)
            .endObject()
            .startObject("object")
            .startObject("properties")
            .startObject("field")
            .field("type", "keyword")
            .endObject()
            .endObject()
            .endObject()
            .startObject("field_that_does_not_match")
            .field("type", "keyword")
            .endObject()
            .endObject()
            .endObject()
            .endObject();
        return createMapperService(mapping);
    }
    private static SearchExecutionContext newSearchExecutionContext(MapperService mapperService) {
        return newSearchExecutionContext(mapperService, null);
    }
    private static SearchExecutionContext newSearchExecutionContext(
        MapperService mapperService,
        BiFunction<MappedFieldType, FieldDataContext, IndexFieldData<?>> indexFieldDataLookup
    ) {
        Settings settings = indexSettings(IndexVersion.current(), 1, 0).put(IndexMetadata.SETTING_INDEX_UUID, "uuid").build();
        IndexMetadata indexMetadata = new IndexMetadata.Builder("test").settings(settings).build();
        IndexSettings indexSettings = new IndexSettings(indexMetadata, settings);