PURE invariant: a provider may list a given channel only once. Returns the friendly error when
existingCount other rows already pair this (ProviderID, ChannelID), else null.
(Defense-in-depth over the DB UQ_AIBridgeProviderChannel UNIQUE constraint.)
PURE invariant: a provider may list a given channel only once. Returns the friendly error when
existingCountother rows already pair this(ProviderID, ChannelID), elsenull. (Defense-in-depth over the DBUQ_AIBridgeProviderChannelUNIQUE constraint.)