setting recordNode value to undefined behaves like delete (#25739)

## Description

This PR updates the set function to behave like delete when record node
values are set "= undefined".

## Breaking Changes

None

Author: daesunp

packages/dds/tree/src/simple-tree/node-kinds/record/recordNodeTypes.ts

@@ -26,6 +26,22 @@ import type { RestrictiveStringRecord } from "../../../util/index.js";
  * Therefore code assigning to these fields must explicitly construct nodes using the schema's constructor or create method,
  * or using some other method like {@link (TreeAlpha:interface).create}.
  *
+ * Field Assignment and Deletion
+ *
+ * Unlike JavaScript's `Record` type, assigning `undefined` to a key in {@link TreeRecordNode} behaves like a `delete` operation,
+ * removing the field's contents.
+ *
+ * This is to stay consistent with {@link TreeObjectNode} behavior.
+ *
+ * Example:
+ * ```ts
+ * const Numbers = schemaFactory.record("Numbers", schemaFactory.number);
+ * const record = new Numbers({ a:1, b:2 });
+ *
+ * // This is equivalent to delete record.a, and removes the entry.
+ * record.a = undefined
+ * ```
+ *
  * @beta
  */
 export interface TreeRecordNode<

packages/dds/tree/src/test/simple-tree/node-kinds/recordNode.spec.ts

@@ -230,6 +230,13 @@ describe("RecordNode", () => {
 				assert.equal(record.baz, 4);
 			});
 
+			it("setting value to undefined behaves as a delete", () => {
+				const record = init(schemaType, { foo: 1 });
+				assert.equal(record.foo, 1);
+				(record as Record<string, number | undefined>).foo = undefined;
+				assert.equal(record.foo, undefined);
+			});
+
 			it("can delete values", () => {
 				const record = init(schemaType, { foo: 1, bar: 2 });
 				assert.equal(record.foo, 1);