fix: optional fields in FailedClosePositionDetails

[alessiocastrica]

@AlexandrosKyriakakis sure, basically when we get a response on a failed order without additional fields (only including error code and message) pydantic validation for FailedClosePositionDetails will fail as those fields were not marked as optional. We can huddle about it if you want more context.

[alessiocastrica]

and a minor fix I added was the fact that cancel_orders should be optional in the method as it's an optional query parameter