Improve type annotations for model classmethods

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.

Search the current definitions of RedisModel.get(), HashModel.get(), JsonModel.get(), and other model-returning classmethods, then compare the approach in closed PR #520 with the current codebase. Update the type annotations so subclass calls infer the concrete model type, and verify that IDE inference and linter checks recognize model attributes correctly.