fix: Reset buffer after sending to server (#1363)

### Purpose
Need to rewind the buffer back to the start for the next endpoint, so we
send the entire data stream

python/langsmith/client.py

@@ -1821,6 +1821,8 @@ def _send_compressed_multipart_req(self, data_stream, *, attempts: int = 3):
         _context: str = ""
 
         for api_url, api_key in self._write_api_urls.items():
+            data_stream.seek(0)
+
             for idx in range(1, attempts + 1):
                 try:
                     headers = {