Fix incorrect swap index in bubble-down loop in select_messages_to_republish

[sudo-shashank]

Summary of changes

Changes introduced in this pull request:

• Fix the incorrect swap index and add test to verify.

Reference issue to close (if applicable)

Closes #7075

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

• Refactor

  • Improved message chain ordering logic during republish operations.

• Tests

  • Added unit tests for message chain reordering functionality.

[coderabbitai]

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	The PR directly addresses issue #7075 by extracting the bubble-down logic into a helper method and fixing the swap index from (i, i+1) to (j, j+1).
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing the bubble-down swap bug and adding necessary tests; no unrelated modifications are present.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title clearly and specifically describes the main bug fix: correcting an incorrect swap index in the bubble-down loop within select_messages_to_republish, which is the primary change throughout the PR.

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✨ 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 shashank/fix-incorrect-swap-index

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch shashank/fix-incorrect-swap-index

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[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.25%. Comparing base (3b7dcd7) to head (f46bf3a).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/message_pool/msg_chain.rs	88.88%	1 Missing ⚠️
src/message_pool/msgpool/republish.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/message_pool/msg_chain.rs	84.79% <88.88%> (+0.09%)	⬆️
src/message_pool/msgpool/republish.rs	87.09% <95.23%> (+4.11%)	⬆️

... and 10 files with indirect coverage changes

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