[v0.20.x-branch] Backport #11032: contractcourt: retain deadline across contest resolution

[github-actions]

Backport of #11032

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Change Description

Outgoing contest resolvers embed the timeout resolver they transition to once
the outgoing HTLC reaches its expiry. Forward the corresponding incoming HTLC
expiry to that embedded resolver so the existing deadline remains available
after the transition.

The added test exercises the contest-to-timeout transition and checks that the
supplied deadline is retained.

Steps to Test

go test ./contractcourt \
  -run 'TestHtlcOutgoingResolver(SupplementDeadline|Timeout|RemoteClaim) \
  -count=1

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the changed behavior are included.

Code Style and Documentation

• The change follows the code documentation and line-length guidelines.
• Commits follow the ideal Git commit structure.
• A change description is included in the 0.21.2 release notes.

[github-actions]

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-11032-to-v0.20.x-branch
git worktree add --checkout .worktree/backport-11032-to-v0.20.x-branch backport-11032-to-v0.20.x-branch
cd .worktree/backport-11032-to-v0.20.x-branch
git reset --hard HEAD^
git cherry-pick -x 5b4c5067dbd3aa0714b8550511b1e05f25220b83
git push --force-with-lease

[github-actions]

🔴 PR Severity: CRITICAL

gh pr view | 3 files | 48 lines changed

🔴 Critical (1 file)

• contractcourt/htlc_outgoing_contest_resolver.go - contractcourt/* handles on-chain dispute resolution and breach/contest logic

🟢 Low (2 files)

• contractcourt/htlc_outgoing_contest_resolver_test.go - test-only change
• docs/release-notes/release-notes-0.20.2.md - release notes documentation

Analysis

The core change modifies contractcourt/htlc_outgoing_contest_resolver.go, which is part of the on-chain contract dispute resolution logic (forward contest deadline handling). This package directly affects breach/contest handling for outgoing HTLCs, so it falls under the CRITICAL tier regardless of the small diff size (8 non-test lines changed). The remaining files are a test addition and a release-notes update, which do not affect the severity determination.

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_{To override, add a severity-override-{critical,high,medium,low} label.}

[ziggie1984]

LGTM (release notes were the only merge conflict)