Describe the bug

According to the language definition,

string(bytes) -> string converts a byte sequence to a UTF-8 string, errors for invalid code points.

However, cel-java does not error for invalid code points.

To Reproduce

string(b"\xff")

Expected behavior

I would expect a runtime error, but the expression evaluates to "�" instead.

Additional context

bytes_to_string calls ByteString::toStringUtf8. It should probably check ByteString::isValidUtf8 first, and raise an error if that returns false.