fix: make diff snapshot export cancellable

[hanabi1224]

Summary of changes

Our ChainStream implementation never returns Poll::Pending so we need some yield point for tokio to decide whether to discard the task. An alternative fix is implemented in #7254
Tested with #7252

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

• Performance
  • Improved responsiveness during export operations by periodically yielding execution while draining large data streams.
• Bug Fixes
  • Reduced the chance of export tasks monopolizing the scheduler and making the app appear unresponsive during long-running operations.

[coderabbitai]

Walkthrough

In do_export's diff-export path, the stream-draining loop now counts try_next() iterations and yields to Tokio every 128 successful reads.

Changes

Async yield in diff-export stream drain loop

Layer / File(s)	Summary
Stream drain loop with yield_now src/tool/subcommands/archive_cmd.rs	Adds a counter to the try_next() drain loop and inserts tokio::task::yield_now().await every 128 iterations, replacing the prior tight loop.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: making diff snapshot export cancellable.
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/make-export-diff-cancellable

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch hm/make-export-diff-cancellable

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_{Comment @coderabbitai help to get the list of available commands.}

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.10%. Comparing base (28d3c1c) to head (1578bab).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/tool/subcommands/archive_cmd.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/tool/subcommands/archive_cmd.rs	29.48% <0.00%> (-0.19%)	⬇️

... and 3 files with indirect coverage changes

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