Fix MemCacheStore local cache duplication

Followup: https://github.com/rails/rails/pull/42833

The previous fix wasn't working in practice because the LocalCache
middleware doesn't use `with_local_cache` but directly set a
regular `LocalStore` instance in the registry.

So instead we redecorate it on every access. It may cause some extra
allocations, but since it only happens in 6.1 mode, it's not as
much of a concern.

activesupport/lib/active_support/cache/mem_cache_store.rb

@@ -7,6 +7,7 @@ rescue LoadError => e
   raise e
 end
 
+require "delegate"
 require "active_support/core_ext/enumerable"
 require "active_support/core_ext/array/extract_options"
 
@@ -33,7 +34,7 @@ module ActiveSupport
       prepend Strategy::LocalCache
 
       module DupLocalCache
-        class LocalStore < Strategy::LocalCache::LocalStore
+        class DupLocalStore < DelegateClass(Strategy::LocalCache::LocalStore)
           def write_entry(_key, entry)
             if entry.is_a?(Entry)
               entry.dup_value!
@@ -42,12 +43,12 @@ module ActiveSupport
           end
 
           def fetch_entry(key)
-            entry = @data.fetch(key) do
+            entry = super do
               new_entry = yield
               if entry.is_a?(Entry)
                 new_entry.dup_value!
               end
-              @data[key] = new_entry
+              new_entry
             end
             entry = entry.dup
 
@@ -59,13 +60,16 @@ module ActiveSupport
           end
         end
 
-        def with_local_cache
-          if ActiveSupport::Cache.format_version == 6.1
-            use_temporary_local_cache(LocalStore.new) { yield }
-          else
-            super
+        private
+          def local_cache
+            if ActiveSupport::Cache.format_version == 6.1
+              if local_cache = super
+                DupLocalStore.new(local_cache)
+              end
+            else
+              super
+            end
           end
-        end
       end
       prepend DupLocalCache