Fix pickling auto_exc exceptions with kw_only fields

[lin-hongkuan]

Summary

Fixes #734.

This adds an attrs-generated __reduce__ for auto_exc=True exception classes that have init=True keyword-only fields. The generated reducer reconstructs the exception without calling __init__, then restores the normal pickle state, so required keyword-only fields work across plain, frozen, slotted, and frozen-slotted exception classes.

The generation is intentionally narrow: it only applies to attrs-generated exception __init__ methods with keyword-only init fields, leaves positional-only exceptions on BaseException.__reduce__, and does not override custom __reduce__ implementations, including inherited ones.

Tests

• PYTHONPATH=src python -m pytest -q
• PYTHONPATH=src python -m ruff check src/attr/_make.py tests/test_functional.py
• PYTHONPATH=src python -m ruff format --check src/attr/_make.py tests/test_functional.py
• git diff --check