[MEDIUM] Q1: Sender-controlled MessageAttributes override SQS system attributes

[ahmedkamalio]

Severity: Medium — verified

• sqs/service.go:243-253 — sender-controlled MessageAttributes are
  merged after SQS system attributes, so a malicious producer can
  overwrite ApproximateReceiveCount and defeat retry/poison-pill/DLQ
  logic.

Fix

Merge system attributes last (or reject/namespace caller keys that collide
with system attributes).

Identified during an internal security review of the Gas framework
(2026-07-03).