Content-boundary hardening suite across inbound surfaces

What this changes for operators

• Operators evaluating OpenClaw against 'is it safe to put agents on real channels' can use this suite as evidence of a threat model, not just a feature list.
• Gateway operators should verify whether gateway.auth.rateLimit was unset in their config — the on-by-default ratelimit changes observable behavior for non-browser/HTTP auth flows.
• Plugin authors should treat allowFrom sender allowlists as the canonical inbound boundary; post-dispatch filtering is the older model.