[v0.20.x-branch] Backport #11190: zpay32: reject duplicate payment hash fields

[github-actions]

Backport of #11190

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Change Description

In this PR, we reject BOLT 11 invoices that contain more than one payment
hash (p) field. zpay32.Decode previously kept the first supported payment
hash and ignored later fields, so the interpreted payment hash depended on
field order.

The decoder now tracks whether a p field has appeared separately from whether
its contents parsed successfully. Any later p field returns
ErrDuplicatePaymentHash. This covers identical hashes, distinct hashes, and a
malformed first field followed by a valid one.

This change is deliberately stricter than the current BOLT 11 text, which
tells a reader to use the first p field; it is motivated by
lightning/bolts#1357.

Note: the same order dependence remains for the other single-occurrence fields
(s, d, m, n, h, x, c, f, 9), which all keep first-seen-wins
behavior. This PR is intentionally limited to the payment hash; the s
(payment address) field looks like the natural candidate for the same treatment
in a follow-up.

Steps to Test

go test ./zpay32
make lint-native

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. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.
• Any new lncli commands have appropriate tags in the comments for the rpc in the proto file.
• There is a change description in the release notes, or [skip ci] in the commit message for small changes.

[github-actions]

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

git fetch origin backport-11190-to-v0.20.x-branch
git worktree add --checkout .worktree/backport-11190-to-v0.20.x-branch backport-11190-to-v0.20.x-branch
cd .worktree/backport-11190-to-v0.20.x-branch
git reset --hard HEAD^
git cherry-pick -x e2f2706367aa6e7a39f8ee14204b43087e27600f
git push --force-with-lease

[ziggie1984]

LGTM

[github-actions]

🟡 PR Severity: MEDIUM

gh pr view | 5 files | 150 lines changed

🟡 Medium (2 files)

• zpay32/decode.go - zpay32 package (BOLT-11 invoice decoding)
• zpay32/invoice.go - zpay32 package (BOLT-11 invoice type)

🟢 Low (3 files)

• docs/release-notes/release-notes-0.20.5.md - release notes
• zpay32/invoice_internal_test.go - test-only change
• zpay32/invoice_test.go - test-only change

Analysis

This is a backport of #‌11190 (reject duplicate payment hashes) to the v0.20.x branch. The substantive change is confined to zpay32/decode.go and zpay32/invoice.go, which fall under the zpay32/* package (MEDIUM tier). The remaining files are tests and release notes. Total non-test/non-generated lines changed (~38) and file count (3) are well below the bump thresholds, and no critical packages are touched, so no severity escalation applies.

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