fix(runway): ISS-014 use portable DLQ failures

[sbalabanov]

Summary

Intent:

• Preserve backend-independent dead-letter failure reasons in Runway responses.
• Keep legacy metadata and the existing generic reason as compatibility fallbacks.

Changes:

• Prefer consumer.Delivery.Failure() when constructing a terminal merge result.
• Add table-driven coverage for portable failures, legacy metadata, default reasons, and published result identity.

Reproduction:

• A non-MySQL queue backend dead-letters a merge request with Delivery.Failure() set to the provider error but without dlq.last_error metadata.
• Runway previously returned the generic failure reason; it now publishes the actual recorded failure to the client.

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Test Plan

AI Verification

Validated at f8d946c on Sep 4 22:21 UTC · 3 files analyzed · 903ms

Validator	Status	Issues
ios-lint	not_applicable	0
web-unit	not_applicable	0
web-repocheck	not_applicable	0
fix-disclosure	not_applicable	0
go-coverage	not_applicable	0
go-gazelle	not_applicable	0
go-thrift-lint	not_applicable	0
java-lint	not_applicable	0
web-lint	not_applicable	0
merge-conflict	not_applicable	0
go-lint	not_applicable	0
visual-ios	not_applicable	0
go-proto-lint	not_applicable	0
android-lint	not_applicable	0
android-coverage	not_applicable	0
uber-one	not_applicable	0
web-typecheck	not_applicable	0
arc-unit	not_applicable	0
arc-lint	not_applicable	0
visual-web	not_applicable	0
java-coverage	not_applicable	0
ios-test	not_applicable	0
web-coverage	not_applicable	0
diff-template	not_applicable	0
visual-android	not_applicable	0
ureview	completed	0
custom	not_applicable	0

0 issues detected

_{Skipped validators: claude · EngWiki}

Issues

T3-ISS-014

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