Support type aliases in union passthrough

[yzhkali]

Summary

• unwrap PEP 695 type aliases when union passthrough checks native union members
• keep spillover handling for non-passthrough union members intact
• add Python 3.12 coverage for a type alias mixed with a dataclass in the target union

Closes #742

Tests

• .venv/bin/python -m pytest tests/strategies/test_tagged_unions_695.py tests/strategies/test_union_passthrough.py tests/strategies/test_union_passthrough_695.py
• .venv/bin/ruff check src/cattrs/strategies/_unions.py tests/strategies/test_union_passthrough_695.py
• .venv/bin/ruff format --check src/cattrs/strategies/_unions.py tests/strategies/test_union_passthrough_695.py
• git diff --check