fix(python): make union case classes hashable again

[caroott]

Fixes #4804

Hashing a union value that contains another union as a field raises TypeError: unhashable type at runtime, e.g.

type Inner = | A
type Outer = | Wrap of Inner

(Wrap A).GetHashCode() // TypeError: unhashable type: 'Inner_A'

The same code works on .NET and JS, and breaks things like List.distinct or using unions as dictionary keys on Python.

The tagged_union decorator applies dataclass() to every case class. Since that generates __eq__ without frozen=True, Python sets __hash__ = None on the class, shadowing the __hash__ the Union base inherits from HashableBase. So case classes were explicitly unhashable, and Union.GetHashCode (hash((self.tag, *self.fields))) blows up as soon as a field is itself a union. The decorator's docstring already claims __hash__ is generated. This restores it after the dataclass() call. It's consistent with the dataclass __eq__: equal values have equal (tag, *fields), hence equal hashes.

Added tests for the hash protocol, hash/eq consistency, nested unions and set membership.

One note: type checkers still consider the case classes unhashable statically (dataclass_transform models them as non-frozen dataclasses), even though __hash__ now exists at runtime.

[dbrattli]

Thanks for fixing!