fix(whatsapp): guard Mexico normalizer on length and clarify WAID wording

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/services/whatsapp/phone_normalizers/mexico_phone_normalizer.rb

@@ -1,10 +1,16 @@
 # Handles Mexico phone number normalization
 #
 # WhatsApp may include a "1" after Mexico's country code (521), while contacts
-# can already be stored without it (52). Normalize to the stored E.164 format.
+# can already be stored without it (52). Normalize to the stored WAID (digits-only)
+# format; provider formatting (whatsapp:+...) happens in the normalization service.
+# The mobile "1" only appears on the 13-digit form (52 + 1 + 10 digits), so guard
+# on length to avoid stripping a legitimate "1" from a shorter national number.
 class Whatsapp::PhoneNormalizers::MexicoPhoneNormalizer < Whatsapp::PhoneNormalizers::BasePhoneNormalizer
+  COUNTRY_CODE_WITH_MOBILE_LENGTH = 13
+
   def normalize(waid)
     return waid unless handles_country?(waid)
+    return waid unless waid.length == COUNTRY_CODE_WITH_MOBILE_LENGTH
 
     waid.sub(/^521/, '52')
   end

spec/services/whatsapp/phone_normalizers/mexico_phone_normalizer_spec.rb

@@ -0,0 +1,23 @@
+require 'rails_helper'
+
+describe Whatsapp::PhoneNormalizers::MexicoPhoneNormalizer do
+  subject(:normalizer) { described_class.new }
+
+  describe '#normalize' do
+    it 'strips the mobile "1" from the 13-digit form (521 + 10 digits)' do
+      expect(normalizer.normalize('5215512345678')).to eq('525512345678')
+    end
+
+    it 'leaves the already-normalized 12-digit form untouched' do
+      expect(normalizer.normalize('525512345678')).to eq('525512345678')
+    end
+
+    it 'does not strip a legitimate leading "1" from a 12-digit national number' do
+      expect(normalizer.normalize('521512345678')).to eq('521512345678')
+    end
+
+    it 'returns non-Mexico numbers unchanged' do
+      expect(normalizer.normalize('5511999998888')).to eq('5511999998888')
+    end
+  end
+end