fix(cmc): retry ICP burns that fail after cycles were already delivered

[Rachit2323]

The problem

When you send ICP to get cycles, CMC does two things: give you the cycles, then burn your ICP. If the burn fails, it just logs it and forgets. You keep the cycles, but your ICP never actually gets burned.
No retry, ever.

What I did

If the burn fails, save it instead of forgetting it. Try again every heartbeat until it works. Never repeat the "give you cycles" part again - only retry the burn.

Testing

Added tests for the new save/retry logic. All existing tests still pass.

[github-actions]

Run on 204431f URL: https://github.com/dfinity/ic/actions/runs/32594520090

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
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1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

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[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 204431f.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f975f1a5-0a47-4a3b-b7f0-a55609e90390?scanId=bd287f15-5f48-43de-96d1-8e431bfb516a&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/nns/cmc/src/main.rs     Add PendingBurn struct, PendingBurns tracking, and retry logic for pending burns ► rs/nns/cmc/unreleased_changelog.md     Update changelog entry: retried failed ICP burns after CMC notification processing