DEV: cleanup custom filters to prevent leaks

Ensures we clean up any custom filters added in the specs to prevent any leaks when running the specs.

Follow up to https://github.com/discourse/discourse/pull/26770#discussion_r1582464760

lib/search.rb

@@ -349,19 +349,19 @@ class Search
   end
 
   def self.advanced_order(trigger, &block)
-    (@advanced_orders ||= {})[trigger] = block
+    advanced_orders[trigger] = block
   end
 
   def self.advanced_orders
-    @advanced_orders
+    @advanced_orders ||= {}
   end
 
   def self.advanced_filter(trigger, &block)
-    (@advanced_filters ||= {})[trigger] = block
+    advanced_filters[trigger] = block
   end
 
   def self.advanced_filters
-    @advanced_filters
+    @advanced_filters ||= {}
   end
 
   def self.custom_topic_eager_load(tables = nil, &block)