### 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
}
              }
            }}}
            """);
        String source = """
            { "id" : "1", "user" : { "first" : "John", "address" : { "city" : "Toronto" } }, "user_account" : { "details" : { "id" : 2 } } }
            """;
        var results = fetchFields(mapperService, source, fieldAndFormatList("*", null, true));
        assertNotNull(results.get("user_account"));
        assertEquals("2", eval(new String[] { "details.id", "0" }, results.get("user_account").getValues().get(0)).toString());
        results = fetchFields(mapperService, source, fieldAndFormatList("user.address.*", null, true));
        assertNotNull(results.get("user"));
        assertNull(eval("first", results.get("user").getValues().get(0)));
        assertEquals("Toronto", eval(new String[] { "address.city", "0" }, results.get("user").getValues().get(0)));
    }
    public void testNestedGrouping() throws IOException {
        MapperService mapperService = createMapperService("""
            { "_doc" : { "properties": {
                                 "age": {
                                   "type": "integer"
                                 },
                                 "loan": {
                                   "type": "keyword"
                                 },
                                 "marital": {
                                   "type": "keyword"
                                 },
                                 "ml": {
                                   "properties": {
                                     "feature_importance": {