TST: Added tests to check if file exists for read_json.
- [x] closes 29102 and the thread in #29104. 
- [x ] tests added / passed
- [x ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

Per the thread in #29104, there should be a differentiation between a json string passed as input and a filepath (from filepath_or_buffer variable passed into _get_data_from_filepath function). If it is not a valid string, then we cannot just raise a FileNotFound Exception error. Therefore, the variable must be checked to see if it is a valid JSON string, if it is a valid file path, and suggest a course of action or raise an appropriate error if it is a faulty string. 

Please open only one pr per topic