fix(jadepy): avoid blocking large serial reads

[Bortlesboat]

Summary

• add a focused regression test for JadeSerialImpl.read()
• read only buffered bytes when data is already waiting instead of always requesting the full chunk
• fall back to a 1-byte blocking read when nothing is buffered

Testing

• python -m unittest discover -s tests -p test_jade_serial.py -v

[jgriffiths]

Please do not submit AI patches without noting you have done so.

Also please provide some kind of rationale for this change.