[ziggie1984]

Backport of #10520

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Change Description

Extend routerrpc.FailureDetail with explicit values for invoice and AMP
validation outcomes and map the corresponding invoice resolution results in
SubscribeHtlcEvents.

This prevents HTLC event subscriptions from terminating with an unknown
failure-resolution error for these outcomes and gives subscribers stable,
specific failure details instead of UNKNOWN.

The original release-note entry is moved from the 0.21.0 notes to the active
0.20.5 notes, and the protobuf artifacts are regenerated with the current
20.x toolchain.

Verification

• make rpc-check
• go test ./lnrpc/...

[ziggie1984]

LGTM needed for #11224

[github-actions]

🟠 PR Severity: HIGH

gh pr view | 5 files | 150 lines changed

🟠 High (4 files)

• lnrpc/routerrpc/router.proto - RPC/API definition change (new field)
• lnrpc/routerrpc/router.pb.go - auto-generated from the proto change above
• lnrpc/routerrpc/router.swagger.json - auto-generated swagger spec from the proto change
• lnrpc/routerrpc/subscribe_events.go - router RPC event subscription logic updated to use the new field

🟢 Low (1 file)

• docs/release-notes/release-notes-0.20.5.md - release notes entry

Analysis

All substantive changes live under lnrpc/routerrpc/*, which falls under the HIGH severity tier (RPC/API definitions). The change adds a field to router.proto and threads it through the regenerated router.pb.go/router.swagger.json and subscribe_events.go. No CRITICAL packages are touched, and excluding the auto-generated router.pb.go, the PR is small (~4 substantive files, ~34 non-generated lines), so no severity bump applies.

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