### 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
List<FieldAndFormat> fieldList = List.of(
                new FieldAndFormat("_id", null),
                new FieldAndFormat("_index", null),
                new FieldAndFormat("_version", null),
                new FieldAndFormat("_routing", null),
                new FieldAndFormat("_ignored", null)
            );
            FieldFetcher fieldFetcher = FieldFetcher.create(
                newSearchExecutionContext(
                    mapperService,
                    (ft, fdc) -> fieldDataLookup(fdc.sourcePathsLookup()).apply(ft, fdc.lookupSupplier(), fdc.fielddataOperation())
                ),
                fieldList
            );
            IndexSearcher searcher = newSearcher(iw);
            LeafReaderContext readerContext = searcher.getIndexReader().leaves().get(0);
            fieldFetcher.setNextReader(readerContext);
            Source s = SourceProvider.fromStoredFields().getSource(readerContext, 0);
            Map<String, DocumentField> fetchedFields = fieldFetcher.fetch(s, 0);
            assertThat(fetchedFields.size(), equalTo(5));
            assertEquals(docId, fetchedFields.get("_id").getValue());
            assertEquals(routing, fetchedFields.get("_routing").getValue());
            assertEquals("test", fetchedFields.get("_index").getValue());
            assertEquals(version, ((Long) fetchedFields.get("_version").getValue()).longValue());
            assertEquals("integer_field", fetchedFields.get("_ignored").getValue());
        });
        
        for (String fieldname : List.of(
            SeqNoFieldMapper.NAME,
            SourceFieldMapper.NAME,