mutable-override does not understand unions

Bug Report
If I enable mutable-override it fails as it does not understand simple unions.

To Reproduce
Turn on mutable-override

class Base:
    val: bool|None = True
    
class Derived(Base):
    val = False

Gist: https://gist.github.com/mypy-play/aeec67e97957ea9e60431ce8466d8af3
Playground: https://mypy-play.net/?mypy=latest&python=3.12&enable-error-code=mutable-override&gist=aeec67e97957ea9e60431ce8466d8af3

Expected Behavior
no error

Actual Behavior

main.py:5: error: Covariant override of a mutable attribute (base class "Base" defined the type as "bool | None", expression has type "bool")  [mutable-override]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.18.2
• Mypy command-line flags: enable-error-code="mutable-override"
• Python version used: 3.12

mutable-override を有効にした mypy 1.18.2 で、提供された Base/Derived の例を使って報告を再現し、その後、union アノテーションに対する mutable-override の型チェックのエントリポイントを追跡します。完了条件は、その例でエラーが発生せず、既存の mutable-attribute override チェックが引き続き機能することです。