Revert "chore: removes unused API surface"

This reverts commit c24dfbd.

Author: baywet

src/Microsoft.OpenApi/Reader/ParseNodes/MapNode.cs

@@ -37,6 +37,19 @@ public MapNode(ParsingContext context, JsonNode node) : base(
             _nodes = _node.Select(p => GetPropertyNodeFromJsonNode(p.Key, p.Value)).ToList();
         }
 
+        public PropertyNode? this[string key]
+        {
+            get
+            {
+                if (_node.TryGetPropertyValue(key, out var value))
+				{
+					return GetPropertyNodeFromJsonNode(key, value);
+				}
+
+                return null;
+            }
+        }
+
         public override Dictionary<string, T> CreateMap<T>(Func<MapNode, OpenApiDocument, T> map, OpenApiDocument hostDocument)
         {
             var jsonMap = _node ?? throw new OpenApiReaderException($"Expected map while parsing {typeof(T).Name}", Context);

src/Microsoft.OpenApi/Reader/V2/OpenApiV2Deserializer.cs

@@ -19,16 +19,19 @@ private static void ParseMap<T>(
             T domainObject,
             FixedFieldMap<T> fixedFieldMap,
             PatternFieldMap<T> patternFieldMap,
-            OpenApiDocument doc)
+            OpenApiDocument doc,
+            List<string>? requiredFields = null)
         {
             if (mapNode == null)
             {
                 return;
             }
 
-            foreach (var propertyNode in mapNode)
+            var allFields = fixedFieldMap.Keys.Union(mapNode.Select(static x => x.Name));
+            foreach (var propertyNode in allFields)
             {
-                propertyNode.ParseField(domainObject, fixedFieldMap, patternFieldMap, doc);
+                mapNode[propertyNode]?.ParseField(domainObject, fixedFieldMap, patternFieldMap, doc);
+                requiredFields?.Remove(propertyNode);
             }
         }
 

test/Microsoft.OpenApi.Tests/Reader/MapNodeTests.cs

@@ -1,5 +1,4 @@
-using System.Linq;
-using System.Text.Json.Nodes;
+using System.Text.Json.Nodes;
 using Microsoft.OpenApi.Reader;
 using Xunit;
 
@@ -15,6 +14,6 @@ public void DoesNotFailOnNullValue()
 
         Assert.NotNull(mapNode);
         Assert.Single(mapNode);
-        Assert.True(mapNode.First().Value.JsonNode.IsJsonNullSentinel());
+        Assert.True(mapNode["key"].Value.JsonNode.IsJsonNullSentinel());
     }
 }