### 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
assertThat(fields.size(), equalTo(1));
        assertThat(fields.keySet(), containsInAnyOrder("field"));
        for (DocumentField field : fields.values()) {
            assertThat(field.getValues().size(), equalTo(1));
            assertThat(field.getValue(), equalTo("some text"));
        }
    }
    public void testSimpleUnmappedFields() throws IOException {
        MapperService mapperService = createMapperService();
        XContentBuilder source = XContentFactory.jsonBuilder()
            .startObject()
            .field("unmapped_f1", "some text")
            .field("unmapped_f2", "some text")
            .field("unmapped_f3", "some text")
            .field("something_else", "some text")
            .nullField("null_value")
            .startObject("object")
            .field("a", "foo")
            .endObject()
            .field("object.b", "bar")
            .endObject();
        Map<String, DocumentField> fields = fetchFields(mapperService, source, fieldAndFormatList("unmapped_f*", null, true));
        assertThat(fields.size(), equalTo(3));
        assertThat(fields.keySet(), containsInAnyOrder("unmapped_f1", "unmapped_f2", "unmapped_f3"));
        fields = fetchFields(mapperService, source, fieldAndFormatList("un*1", null, true));
        assertThat(fields.size(), equalTo(1));
        assertThat(fields.keySet(), containsInAnyOrder("unmapped_f1"));
        fields = fetchFields(mapperService, source, fieldAndFormatList("*thing*", null, true));
        assertThat(fields.size(), equalTo(1));
        assertThat(fields.keySet(), containsInAnyOrder("something_else"));