feat: add observability logging for invalid MAC addresses

- Log when normalize_mac drops invalid MAC addresses from SNMP data
- Helps diagnose data quality issues from network equipment

lib/towerops/gaiia/subscriber_matching.ex

@@ -460,13 +460,19 @@ defmodule Towerops.Gaiia.SubscriberMatching do
   defp normalize_mac(nil), do: nil
 
   defp normalize_mac(mac) do
-    mac
-    |> String.downcase()
-    |> String.replace(~r/[^0-9a-f]/, "")
-    |> case do
+    normalized =
+      mac
+      |> String.downcase()
+      |> String.replace(~r/[^0-9a-f]/, "")
+
+    case normalized do
       <<a::binary-2, b::binary-2, c::binary-2, d::binary-2, e::binary-2, f::binary-2>> ->
         "#{a}:#{b}:#{c}:#{d}:#{e}:#{f}"
 
+      invalid when invalid != "" ->
+        Logger.debug("Dropping invalid MAC address from SNMP data: #{inspect(mac)} (normalized: #{invalid})")
+        nil
+
       _ ->
         nil
     end