lnwallet+lnwire: handle boundary cases

[yyforyongyu]

Change Description

Make channel funding completion return both results when its pending wallet
reservation is no longer present. Define the last block height for zero-length
query and reply channel ranges while preserving existing non-empty and
overflow behavior.

Add focused regression coverage and a v0.21.3 release note.

Steps to Test

• go test ./lnwallet ./lnwire -count=1
• go test -race ./lnwallet -run TestHandleFundingCounterPartySigsMissingReservation -count=1
• go test -race ./lnwire -run TestChannelRangeLastBlockHeight -count=1

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial.
• The change obeys the code documentation and commenting guidelines, and lines wrap at 80.
• Commits follow the ideal Git commit structure.
• No new logging statements are introduced.
• No new lncli commands are introduced.
• There is a change description in the release notes.

Please see the contribution guidelines for further guidance.

[github-actions]

🔴 PR Severity: CRITICAL

gh pr view | 6 files | 169 lines changed

🔴 Critical (3 files)

• lnwallet/wallet.go - wallet operations / channel funding logic
• lnwire/query_channel_range.go - Lightning wire protocol message
• lnwire/reply_channel_range.go - Lightning wire protocol message

🟢 Low (3 files)

• docs/release-notes/release-notes-0.21.3.md - release notes only
• lnwallet/wallet_test.go - test-only change
• lnwire/reply_channel_range_test.go - test-only change

Analysis

This PR touches both lnwallet/* (wallet operations) and lnwire/* (wire protocol message encoding/decoding for channel range queries), both of which are CRITICAL-tier packages. The core change is small (1 line in wallet.go, ~8 lines across the two lnwire files) and is accompanied by good test coverage, but changes to wire message encoding and wallet logic warrant expert review given their correctness sensitivity and potential for peer-interop or fund-safety impact.

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