fix: prevent nil assignment for membership_fee_type_id

Reject attempts to set membership_fee_type_id to nil when a current type
exists.

lib/membership_fees/changes/validate_same_interval.ex

@@ -42,9 +42,9 @@ defmodule Mv.MembershipFees.Changes.ValidateSameInterval do
       is_nil(current_type_id) ->
         changeset
 
-      # If new type is nil, that's allowed (removing type)
+      # If new type is nil, reject the change (membership_fee_type_id is required)
       is_nil(new_type_id) ->
-        changeset
+        add_nil_type_error(changeset)
 
       # Both types exist - validate intervals match
       true ->
@@ -125,6 +125,17 @@ defmodule Mv.MembershipFees.Changes.ValidateSameInterval do
     )
   end
 
+  # Add validation error when trying to set membership_fee_type_id to nil
+  defp add_nil_type_error(changeset) do
+    message = "Cannot remove membership fee type. A membership fee type is required."
+
+    Ash.Changeset.add_error(
+      changeset,
+      field: :membership_fee_type_id,
+      message: message
+    )
+  end
+
   # Format interval atom to human-readable string
   defp format_interval(:monthly), do: "monthly"
   defp format_interval(:quarterly), do: "quarterly"