fix: yield to runtime on every 128 ChainStream polls

[hanabi1224]

Summary of changes

Alternative fix to #7253
Tested with #7252

Bug: when the chain stream is consumed in a tight loop with no yield point inside, the task cannot be cancelled or rescheduled by tokio.

forest/src/tool/subcommands/archive_cmd.rs

Line 601 in 0ba9a6b

while stream.try_next().await?.is_some() {}

let mut stream = stream_chain(...);
while stream.try_next().await?.is_some() {}
stream.into_seen()

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

Summary by CodeRabbit

• Bug Fixes
  • Improved responsiveness of long-running chain stream processing by periodically yielding control back to the runtime. This helps cancellation take effect sooner and improves overall scheduling progress during extended operations.

[coderabbitai]

Walkthrough

ChainStream gains an n_polled: usize counter initialized to 0. poll_next now increments the counter on each call and, every 128 polls, wakes the task and returns Poll::Pending.

Changes

ChainStream periodic yielding

Layer / File(s)	Summary
n_polled counter and yield logic src/ipld/util.rs	Adds n_polled: usize to ChainStream, initializes it in stream_chain, and updates poll_next to wake and return Poll::Pending on every 128th poll.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: ChainStream now yields back to the runtime periodically during polling.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch hm/chain-stream-yield-every-100-polls

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch hm/chain-stream-yield-every-100-polls

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