stubgen: handle PEP 604 unions in inspection mode (#21689)

[apoorvdarshan]

Fixes #21689.

stubgen --inspect-mode crashes with AttributeError on any function whose signature uses PEP 604 union syntax (X | Y):

def process(value: int | str) -> float | None: ...

File ".../mypy/stubgenc.py", line 775, in get_type_fullname
    typename = getattr(typ, "__qualname__", typ.__name__)
AttributeError: 'types.UnionType' object has no attribute '__name__'

Root cause: InspectionStubGenerator.get_type_fullname() assumes the object is a class and reads __qualname__/__name__, but a runtime types.UnionType (the value of int | str) has neither.

Fix: render a UnionType by recursively formatting its members and joining with |, mapping NoneType to None so e.g. float | None renders correctly.

Added a _inspect test in test-data/unit/stubgen.test (fails on master, passes here). stubgen suite green (the one unrelated testAttrsClass_semanal failure is pre-existing in my environment), mypy self-check and ruff clean.

Disclosure: prepared with AI assistance; reviewed and verified locally.