feat(strands-py-wasm): make variant arms inherit from container

[chaynabors]

Hey Chay, wanted to chat about pgrayy/wasm-deps@983c56c#diff-e4f544b88abae11d378b0216c10391b30e49ef793d62aa21dd66abdc445517beR301

One thing I noticed is that since the arms only inherit from _WitVariantCase, we can't really use the parent class as a type on our end. This means that things like isinstance checks or referencing in type signatures don't work because there's no inheritance relationship between the arms and the parent.

Fixed.