Add missing exports for try_cast, bitwise_not, and label

[veeceey]

Summary

Added missing exports for three functions that were defined in sqlmodel/sql/expression.py but not exported in sqlmodel/__init__.py, making them inaccessible to users.

Changes

Added exports for:

• try_cast: SQL TRY_CAST operation for safe type casting (returns NULL on cast failure instead of raising an error)
• bitwise_not: Bitwise NOT operation for integer columns
• label: Create labeled expressions for query results

Why

These functions are fully implemented in sqlmodel/sql/expression.py but were missing from the public API. Users who needed these operations had to import directly from the internal module or use SQLAlchemy directly.

Test plan

• ✅ Verified imports work: from sqlmodel import try_cast, bitwise_not, label
• ✅ Existing tests pass
• These functions are wrappers around SQLAlchemy functions with proper type hints

Related

This makes SQLModel's API more complete and consistent with SQLAlchemy's functionality.

Example usage after this change:

from sqlmodel import select, try_cast, Integer

# Safe casting that returns NULL on failure
statement = select(try_cast(column, Integer))