✨ Add Engine class re-export from SQLAlchemy

[AlefRP]

Pull Request

Description

This PR re-exports SQLAlchemy's Engine class from the top-level sqlmodel
namespace, alongside the already-exported create_engine, create_mock_engine
and engine_from_config.

This makes it possible to import Engine directly from sqlmodel and use it
for type hints when writing your own engine/session helpers, mirroring how
SQLAlchemy exposes it:

from sqlmodel import Engine, create_engine, Session

def make_engine(url: str) -> Engine:
    return create_engine(url)

def get_session(engine: Engine) -> Session:
    return Session(engine)

The export is added next to the other sqlalchemy.engine imports in
sqlmodel/init.py, following the existing import X as X re-export
convention used throughout the file (so it is picked up as public API by type
checkers and linters).

Checklist

• This PR is an obvious typo fix, or it links to a GitHub Discussion for the proposed code change.
• I added tests for the change.
• The new or updated tests fail on the main branch and pass on this PR.
• Coverage stays at 100%.
• The documentation explains the change if needed.

[AlefRP]

Please add the feature label to pass the label check workflow.

[YuriiMotov]

I think we should do it in a more complex way - add not just Engine, but other missing re-exports as well.

Let's start from discussion as it's asked in the PR template