fix(typing): export SQLModelConfig and allow ConfigDict for model_config (#2091)

[DYNOSuprovo]

Summary

Fixes #2091.

Problem

In sqlmodel, SQLModelMetaclass declared model_config: SQLModelConfig.

1. SQLModelConfig was only defined in sqlmodel._compat and was not re-exported by sqlmodel or sqlmodel.main, preventing users from publicly importing or referencing SQLModelConfig.
2. Because SQLModelConfig inherits from Pydantic's BaseConfig/ConfigDict, assigning a standard Pydantic ConfigDict or a plain dictionary literal to model_config on an SQLModel subclass triggered strict type errors in type checkers such as Astral's ty:

   invalid-assignment: Object of type ConfigDict is not assignable to attribute model_config of type SQLModelConfig
   

Solution

• Re-export SQLModelConfig and Pydantic's ConfigDict in sqlmodel and sqlmodel.main.
• Widen SQLModelMetaclass.model_config annotation to SQLModelConfig | ConfigDict | dict[str, Any].
• Add comprehensive unit tests in tests/test_pydantic/test_model_config.py verifying SQLModelConfig and ConfigDict public exports, type validity, and configuration behavior across models.

Verification

• pytest tests/test_pydantic/test_model_config.py: 6 passed
• ty check sqlmodel: 0 diagnostics
• ty check tests/test_pydantic/test_model_config.py: 0 diagnostics
• ruff check sqlmodel tests: passed
• ruff format sqlmodel tests --check: 171 files left unchanged