Fix PrivateAttr defaults not initialized when loading from DB

[veeceey]

Summary

Fixes fastapi#149 - Pydantic.PrivateAttr default and default_factory are ignored by SQLModel.

When SQLAlchemy loads objects from the database, it bypasses __init__ and only calls __new__. The init_pydantic_private_attrs function was setting __pydantic_private__ to None, which caused accessing any PrivateAttr to raise:

TypeError: 'NoneType' object is not subscriptable

Changes

• sqlmodel/_compat.py: Updated init_pydantic_private_attrs() to properly iterate over __private_attributes__ and populate __pydantic_private__ with default values, matching Pydantic's own init_private_attributes behavior. Also fixed the same issue in sqlmodel_table_construct() where __pydantic_private__ was set to None when __pydantic_post_init__ is falsy.

• tests/test_private_attr.py: Added 6 comprehensive tests covering:

  • PrivateAttr(default=...) on direct construction
  • PrivateAttr(default_factory=...) on direct construction
  • PrivateAttr(default=...) after loading from DB
  • PrivateAttr(default_factory=...) after loading from DB
  • Property access pattern over PrivateAttr after DB load
  • PrivateAttr() without defaults is correctly excluded from the dict

Before this fix

class Hero(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    secret_name: str
    _name: str = PrivateAttr(default="default_hero")

# Loading from DB:
hero = session.exec(select(Hero)).first()
hero._name  # TypeError: 'NoneType' object is not subscriptable
hero.__pydantic_private__  # None

After this fix

hero = session.exec(select(Hero)).first()
hero._name  # "default_hero"
hero.__pydantic_private__  # {'_name': 'default_hero'}

Test plan

• All 81 existing core tests pass (no regressions)
• 6 new tests for PrivateAttr behavior all pass
• Verified fix with PrivateAttr(default=...), PrivateAttr(default_factory=...), and PrivateAttr() (no default)
• Verified the property access pattern from issue comments works correctly