fix: prevent async functional API from double-consuming interrupt resume values

[abhigyan631]

Fix for issue #6660.

In async mode, both parent and child scratchpads were capturing references to the same resume tuple in pending_writes. When both attempted to consume the value, it caused iterations to be skipped (e.g., output 1, 123, 12345 instead of expected 1, 12, 123, 1234, 12345).

The fix ensures child scratchpads always delegate to parent for null_resume, preventing the double-consumption race condition. Only the parent scratchpad now 'owns' the resume value lookup.

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[hinthornw]

Do you happen to have a test case that reproduces this?

[abhigyan631]

Do you happen to have a test case that reproduces this?

Yes, I've added a regression test case, test_repro_issue_6660.py, that reproduces the reproduction scenario (5 async tasks/resumes).