fix(func): preserve nested entrypoint interrupt resume behavior

[Giulio Leone (giulio-leone)]

Summary

• treat checkpoint_id=None as an unpinned nested resume so completed child tasks are reused
• avoid surfacing duplicate bubbled GraphInterrupt payloads from nested call futures
• add sync and async regressions for nested entrypoint -> entrypoint interrupt/resume behavior

Testing

• NO_DOCKER=true uv run --python 3.12 pytest tests/test_pregel.py -k "nested_entrypoint_interrupt_resume_reuses_tasks or multiple_subgraphs_mixed_entrypoint" -q
• NO_DOCKER=true uv run --python 3.12 pytest tests/test_pregel_async.py -k "nested_entrypoint_interrupt_resume_reuses_tasks or multiple_subgraphs_mixed_entrypoint" -q
• uv run --python 3.12 python ... (raw issue reproducer now surfaces one interrupt and resumes to foo bar baz qux)

Fixes #6792

[Giulio Leone (giulio-leone)]

Friendly ping — I reproduced the reported nested-entrypoint interrupt/resume bug, validated the fix on the available local backends, and the branch is current. Ready for review whenever convenient; happy to address any feedback. 🙏

[Sydney Runkle (sydney-runkle)]

Hi! I think now out of date w/ main, could you please rerun w/ the new changes? I think my diff might cover this use case...

Thanks for the contribution!

[Giulio Leone (giulio-leone)]

Rebased this branch onto current main and adapted the nested resume fix to the newer replay semantics (checkpoint_id=None should behave as an unpinned resume, not as replay).

Validation on top of current main:

• NO_DOCKER=true uv run --python 3.12 pytest tests/test_pregel.py -k "nested_entrypoint_interrupt_resume_reuses_tasks or multiple_subgraphs_mixed_entrypoint" -q
• NO_DOCKER=true uv run --python 3.12 pytest tests/test_pregel_async.py -k "nested_entrypoint_interrupt_resume_reuses_tasks or multiple_subgraphs_mixed_entrypoint" -q
• git diff --check

Results:

• sync target: 6 passed
• async target: 4 passed

This should clear the staleness introduced by the 15 commits that landed on main after the original PR was opened.