Preserve literal form field names when converting to JSON

Use an object with a null prototype so inherited property names are not
mistaken for existing fields and __proto__ remains a literal JSON key.
Cover __proto__, constructor, and toString with regression cases.

apps/yaak-client/lib/requestBodyConversion.test.ts

@@ -92,6 +92,21 @@ describe("convertRequestBody", () => {
     });
   });
 
+  test.each(["__proto__", "constructor", "toString"])(
+    "preserves the form field %s when converting to JSON",
+    (name) => {
+      const body = convertRequestBody({
+        fromBodyType: BODY_TYPE_FORM_URLENCODED,
+        toBodyType: BODY_TYPE_JSON,
+        body: { form: [{ enabled: true, name, value: "hello" }] },
+      });
+
+      expect(body).toEqual({
+        text: JSON.stringify({ [name]: "hello" }, null, 2),
+      });
+    },
+  );
+
   test("preserves text when converting to form bodies cannot build form pairs", () => {
     const body = convertRequestBody({
       fromBodyType: BODY_TYPE_XML,

apps/yaak-client/lib/requestBodyConversion.ts

@@ -161,7 +161,7 @@ function formBodyToUrlEncodedText(form: unknown[]): string {
 }
 
 function formBodyToObject(form: unknown[]) {
-  const result: Record<string, unknown> = {};
+  const result: Record<string, unknown> = Object.create(null);
 
   for (const pair of form) {
     if (!isRecord(pair)) continue;