What Problem This Solves

Generated media can be delivered twice when a model both sends it through the message tool and returns the same media in its completion payload. Legacy global delivery telemetry also cannot safely identify which route carried a payload when a run targets multiple routes.

Fixes #88307.

Why This Change Was Made

Delivery deduplication now compares completion payloads against route-scoped message-tool evidence. Account fallback is resolved only when routing did not already select an account and the channel plugin exposes account enumeration. Ambiguous global-only evidence is deliberately ignored for multi-route runs so valid payloads are not dropped.

User Impact

Users no longer receive duplicate generated-media handoffs on a single route. Multi-route deliveries retain payloads unless evidence proves the same payload was already sent on that route.

Evidence

• 691 focused tests passed across delivery, outbound payload, gateway, command, and auto-reply suites in Blacksmith Testbox tbx_01kxc3z4kwtx37jb1xjtd7gp1y.
• Fresh gpt-5.6-sol high autoreview reported no accepted or actionable findings after the regression fixes.
• git diff --check passes.
• The broad changed gate was attempted remotely, but the Testbox full-sync classified 2,568 unrelated files and hit the current @smithy/node-http-handler shrinkwrap override mismatch. The exact focused touched-surface proof above is green; exact-head GitHub CI remains required before merge.