JsonProvider writes checkpoints with encoding="utf-8" and the runtime
serialises non-ASCII text verbatim, but the checkpoint CLI still opened
them with the platform default encoding. On Windows (cp1252) `crewai
checkpoint info` raised UnicodeDecodeError and `list` showed a 0-byte
entry for any checkpoint containing non-ASCII text. Open the files as
UTF-8 and add regression tests for the three readers.