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

Issues

T3-ISS-014

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