### Elasticsearch Version

8.7.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

whatever

### Problem Description

Enable TSDB and disable source.

### Steps to Reproduce

```
curl -uelastic:password -HContent-Type:application/json -XPUT localhost:9200/test?pretty -d'{
  "settings": {
    "mode": "time_series",
    "routing_path": "foo"
  },
  "mappings": {
    "_source": {
      "enabled": false
    }
  }
}'
```

The error message you get is `Cannot set both [mode] and [enabled] parameters` but we never tried to set `mode`. I think maybe we want `time series indices only support synthetic source` like we give if you try to disable _source in tsdb.

### Logs (if relevant)

_No response_
server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java
super(NAME, false, enabled, false, TextSearchInfo.NONE, Collections.emptyMap());
        }
        @Override
        public String typeName() {
            return CONTENT_TYPE;
        }
        @Override
        public ValueFetcher valueFetcher(SearchExecutionContext context, String format) {
            throw new UnsupportedOperationException("Cannot fetch values for internal field [" + name() + "].");
        }
        @Override
        public Query existsQuery(SearchExecutionContext context) {
            throw new QueryShardException(context, "The _source field is not searchable");
        }
        @Override
        public Query termQuery(Object value, SearchExecutionContext context) {
            throw new QueryShardException(context, "The _source field is not searchable");
        }
    }
    
    private final @Nullable Mode mode;
    private final Explicit<Boolean> enabled;
    
    private final boolean complete;
    private final String[] includes;
    private final String[] excludes;
    private final SourceFilter sourceFilter;
    private final IndexMode indexMode;
    private SourceFieldMapper(Mode mode, Explicit<Boolean> enabled, String[] includes, String[] excludes, IndexMode indexMode) {
        super(new SourceFieldType((enabled.explicit() && enabled.value()) || (enabled.explicit() == false && mode != Mode.DISABLED)));