Provider messaging readiness is a relationship test

Before enabling provider messaging, prove with read-only calls that the runtime credential authorizes the chosen sender, the sender maps to the intended line, the application allowlist covers it, and an active subscription covers the required inbound event and API version. Install configuration while inert, verify deployed readback, enable the exact-match master flag last, and disable it first on rollback. A controlled canary is still required for delivery and persistence.