fix(twilio): fall back to phone lookup when MessagingServiceSid is unknown

Twilio includes `MessagingServiceSid` on inbound webhooks whenever the
number is part of a Messaging Service pool on Twilio's side (required
for US A2P 10DLC), even for Chatwoot channels registered by phone number
alone. The signature-verify concern was rejecting those webhooks with
403 because lookup-by-MSS returned nil and did not fall through.

Only reject outright when the MSS does resolve to a channel but the
AccountSid mismatches — the cross-tenant guard from the earlier review
is preserved.

app/controllers/concerns/twilio_signature_verify_concern.rb

@@ -52,9 +52,11 @@ module TwilioSignatureVerifyConcern
   def find_twilio_channel
     if params[:MessagingServiceSid].present?
       channel = ::Channel::TwilioSms.find_by(messaging_service_sid: params[:MessagingServiceSid])
-      return channel if channel.present? && (params[:AccountSid].blank? || channel.account_sid == params[:AccountSid])
+      if channel.present?
+        return channel if params[:AccountSid].blank? || channel.account_sid == params[:AccountSid]
 
-      return nil
+        return nil
+      end
     end
     return if params[:AccountSid].blank?