- SendService.selectSendAccount now delegates to AccountSelector with
extractStrategy(channelStrategy) instead of an inline filter that only
checked tenant/status, restoring risk/online/heartbeat checks.
- ConversationService.bindAccount validates the account exists, belongs
to the same tenant, and matches the conversation channel type before
creating the binding.
- Add unit tests for ConversationService.bindAccount validation and
SendService.resolveConversationTarget strategy/health behaviors.