Improve type annotations for model classmethods

[abrookins]

Summary

The get() classmethod on RedisModel, HashModel, and JsonModel returns a generic type that doesn't properly reflect the actual model subclass. This breaks IDE type inference and linter checks.

Current behavior

class User(HashModel):
    name: str

user = await User.get("123")
# IDE thinks `user` is `Model`, not `User`
# user.name doesn't autocomplete

Expected behavior

user = await User.get("123")
# IDE knows `user` is `User`
# user.name autocompletes correctly

Technical details

Current signature:

async def get(cls: Type["Model"], pk: Any) -> "Model"

Should use proper TypeVar binding so the return type matches the calling class:

async def get(cls: Type[_T], pk: Any) -> _T

This pattern should be applied consistently to:

• RedisModel.get()
• HashModel.get()
• JsonModel.get()
• Any other classmethods that return model instances (e.g., save() returning self)

Prior work

PR #520 attempted this fix but was closed due to staleness. The approach was correct but needs to be updated for the current codebase.