stubtest: don't flag __class_getitem__ as missing on a generic stub

[ShirGanon]

Fixes #21253

A class that implements __class_getitem__ at runtime (to make itself subscriptable) is correctly modelled in a stub by declaring the class as generic — either with PEP 695 syntax (class C[T]) or Generic[T]. stubtest, however, reported a false positive in that case:

error: C.__class_getitem__ is not present in stub

verify_typeinfo iterates the runtime class members and, finding __class_getitem__ on the runtime class with no counterpart in the (generic) stub, flagged it as missing.

Change

In verify_typeinfo, skip the __class_getitem__ runtime member when the stub class is generic (TypeInfo.is_generic()) and does not explicitly declare it — the generic type parameters already express that subscriptability at the type level. A non-generic stub that omits a runtime __class_getitem__ is still reported as before, so the change only suppresses the false positive.

Testing

• Added test_class_getitem_on_generic in mypy/test/teststubtest.py with two cases:
  • generic stub (Generic[T]) + runtime __class_getitem__ → no error;
  • non-generic stub + runtime __class_getitem__ → still reports __class_getitem__ is not present in stub.
• Verified the new test fails without the stubtest.py change and passes with it.
• Full mypy/test/teststubtest.py suite passes (68 tests), no regressions.
• black, ruff, and mypy self-check on mypy/stubtest.py are clean.

(I used the Generic[T] form in the test rather than PEP 695 class C[T] so the stub parses on all supported Python versions; the fix covers both, since both make TypeInfo.is_generic() true.)

_{Disclosure: drafted with Claude Code; reviewed, understood, and tested locally.}