SQLAlchemy adapter: support SQLAlchemy 2.0 DeclarativeBase models and typed Select return

[jmartinhoj]

Problem

The get_query function in the SQLAlchemy adapter has two related type-safety issues:

1. SQLAlchemy 2.0 DeclarativeBase not supported

SQLAlchemy 2.0 introduced a new base class DeclarativeBase (replacing the legacy declarative_base() / DeclarativeMeta approach). Models defined with the new style:

from sqlalchemy.orm import DeclarativeBase

class Base(DeclarativeBase):
    pass

class Resource(Base):
    __tablename__ = "resource"
    id: Mapped[int] = mapped_column(primary_key=True)

...do not inherit from DeclarativeMeta, so passing them as table to get_query currently raises an error on type checkers.

2. Bare Select return type loses type information

The return type is currently annotated as Select (unparameterised), which loses row-type information. Callers cannot benefit from type inference on query results.

Proposed fix

• Try-import DeclarativeBase from sqlalchemy.orm (available in SQLAlchemy ≥ 2.0), falling back to a placeholder class for 1.4 compatibility, and include type[DeclarativeBase] in GenericTable.
• Add @overload signatures for get_query so that when an ORM model class is passed, the return type resolves to Select[Tuple[T]], and when a core Table is passed it returns Select[Any].
• Add from __future__ import annotations to keep the subscripted Select[...] syntax compatible with Python 3.8/SQLAlchemy 1.4.

I have a working patch ready and am happy to submit a PR once there is agreement on the approach.