Fix serialization error when caching get_all results

Remove default_proc from result hashes in active_record, mongo, and sequel
adapters before returning from get_all. The default_proc is used as a
convenience during hash construction but prevents Marshal.dump serialization
when results are cached by ActiveSupportCacheStore.

Co-Authored-By: Claude Haiku 4.5 <[email protected]>

Author: jnunemaker

lib/flipper/adapters/active_record.rb

@@ -123,6 +123,7 @@ def get_all(**kwargs)
           features.each do |feature|
             result[feature.key] = result_for_gates(feature, grouped_gates[feature.key])
           end
+          result.default_proc = nil
           result
         end
       end

lib/flipper/adapters/mongo.rb

@@ -129,7 +129,7 @@ def read_many_features(features)
         docs = find_many(features.map(&:key))
         result = {}
         features.each do |feature|
-          result[feature.key] = result_for_feature(feature, docs[feature.key])
+          result[feature.key] = result_for_feature(feature, docs.fetch(feature.key, {}))
         end
         result
       end
@@ -146,7 +146,7 @@ def find(key)
 
       def find_many(keys)
         docs = @collection.find(_id: { '$in' => keys }).to_a
-        result = Hash.new { |hash, key| hash[key] = {} }
+        result = {}
         docs.each do |doc|
           result[doc['_id']] = doc
         end

lib/flipper/adapters/sequel.rb

@@ -117,6 +117,7 @@ def get_all(**kwargs)
         features.each do |feature|
           result[feature.key] = result_for_feature(feature, grouped_db_gates[feature.key])
         end
+        result.default_proc = nil
         result
       end