Support sparse lists and maps in MCP

Author: adwsingh

core/src/main/java/software/amazon/smithy/java/core/serde/document/DocumentParser.java

@@ -97,12 +97,16 @@ public <T> void writeList(Schema schema, T state, int size, BiConsumer<T, ShapeS
                 elementParser.result = null;
             }
         });
+        var schemaToUse = schema;
+        if (schema.type() == ShapeType.DOCUMENT) {
+            schemaToUse = Documents.LIST_SCHEMA;
+        }
         consumer.accept(state, serializer);
         // Note that neither here nor between values are we testing to ensure the value is not null. That's because
         // this method is used primarily with documents that have no schema, and therefore cannot specify that a
         // list is sparse. Rather than drop null values, they are retained.
         elements.add(elementParser.result);
-        setResult(new Documents.ListDocument(schema, elements));
+        setResult(new Documents.ListDocument(schemaToUse, elements));
     }
 
     @Override

core/src/main/java/software/amazon/smithy/java/core/serde/document/Documents.java

@@ -231,7 +231,13 @@ public void serializeContents(ShapeSerializer serializer) {
             serializer.writeMap(schema, members, members.size(), (members, s) -> {
                 var key = schema.mapKeyMember();
                 for (var entry : members.entrySet()) {
-                    s.writeEntry(key, entry.getKey(), entry.getValue(), Document::serialize);
+                    s.writeEntry(key, entry.getKey(), entry.getValue(), (doc, ser) -> {
+                        if (doc == null) {
+                            ser.writeNull(schema.mapValueMember());
+                        } else {
+                            doc.serialize(ser);
+                        }
+                    });
                 }
             });
         }