gh-155053: Fix GenericAlias crash on OOM

[BHUVANSH855]

Summary

Fix a NULL dereference in Objects/genericaliasobject.c when tuple_extend()
fails during TypeVarTuple substitution under allocation failure.

When _PyTuple_Resize() fails, it clears its out-parameter, so subargs
becomes NULL. The error path in subs_tvars() must not call
Py_DECREF(subargs) and should instead propagate the MemoryError.

Also add a regression test covering the allocation-failure path using
_testcapi.set_nomemory() to ensure a MemoryError is raised instead of
crashing the interpreter.

Issue: gh-155053