[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

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[BHUVANSH855]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

[BHUVANSH855]

@picnixz, gentle ping

[BHUVANSH855]

@picnixz,
PR was having merge time conflicts, so i updated the branch and now it seems like, the changes made by me are already in main, after solving conflicts only NEWS entry file remains.
I think closing this PR will be nice.