pd.read_json May Not Maintain Numeric String Index
```python
>>> df = pd.DataFrame(range(3), index=list("123"))
>>> df.to_json(orient="split")
'{"columns":[0],"index":["1","2","3"],"data":[[0],[1],[2]]}'
>>> pd.read_json(df.to_json(orient="split"), orient="split").index
Int64Index([1, 2, 3], dtype='int64')
```

Note that the string nature of the values should be preserved via roundtrip here, but ends up being lossy anyway. Noted during refactor of #28510