Apply `_simple_default` serialization on dict keys to resolve this exception

```python
Traceback (most recent call last):
  File "/Users/brendan/.local/share/virtualenvs/core-bRkH1-KB/lib/python3.11/site-packages/langsmith/client.py", line 281, in _dumps_json_single
    return orjson.dumps(
           ^^^^^^^^^^^^^
TypeError: Dict key must a type serializable with OPT_NON_STR_KEYS

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/brendan/.local/share/virtualenvs/core-bRkH1-KB/lib/python3.11/site-packages/langsmith/client.py", line 5802, in _tracing_thread_handle_batch
    client.multipart_ingest_runs(create=create, update=update, pre_sampled=True)
  File "/Users/brendan/.local/share/virtualenvs/core-bRkH1-KB/lib/python3.11/site-packages/langsmith/client.py", line 1675, in multipart_ingest_runs
    valb = _dumps_json(value)
           ^^^^^^^^^^^^^^^^^^
  File "/Users/brendan/.local/share/virtualenvs/core-bRkH1-KB/lib/python3.11/site-packages/langsmith/client.py", line 321, in _dumps_json
    return _dumps_json_single(obj, _serialize_json)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/brendan/.local/share/virtualenvs/core-bRkH1-KB/lib/python3.11/site-packages/langsmith/client.py", line 292, in _dumps_json_single
    result = json.dumps(
             ^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
TypeError: keys must be str, int, float, bool or None, not PosixPath
```