feat(serialization): add built-in SQLAlchemy DeclarativeBase serializer
SO
2d ago+834+834+012345678901234567890123456789-24-24-01234567890123456789
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PR Title
Add built-in serialization support for SQLAlchemy DeclarativeBase models
Description
This PR adds built-in serialization support for SQLAlchemy models inheriting from DeclarativeBase.
Previously, SQLAlchemy models required custom serializer registration to serialize loaded relationships safely. The new serializer includes loaded columns and relationships while omitting unloaded or expired attributes, preventing implicit lazy-loading errors such as DetachedInstanceError and MissingGreenlet.
The change preserves the existing SQLModel-specific serializer and adds regression tests and documentation explaining relationship serialization and eager loading.
Closes #7725