fix(webidl): honor initializer iterators on platform objects

Use sequence/record discrimination for native Headers, URLSearchParams,
FormData, and URL initializers. Read the author iterator once, preserve
its result or exception, and use own record properties when it is absent.

Cover Request and Response headers, enumerable iterator symbols, and
FormData File conversion. Remove unused imports and the URL
stringification helper.

moli-renderer-v8/src/script_vm/tests/webidl_fetch.rs

@@ -10696,3 +10696,40 @@ fn clipboard_item_record_converts_thenables_before_later_descriptors() {
         .unwrap();
     assert_eq!(result, "ok");
 }
+
+#[test]
+fn webidl_initializer_unions_convert_platform_objects_without_iterators_as_records() {
+    let mut vm = new_storage_test_vm("https://initializer-record.test/");
+    let result = vm.eval(r#"
+(() => {
+  for (const Constructor of [Headers, URLSearchParams]) {
+    const url = new URL('https://initializer-record.test/path?query=value');
+    Object.defineProperty(url, Symbol.toPrimitive, {value() {
+      throw new Error('URL object must use record conversion');
+    }});
+    if (Array.from(new Constructor(url)).length !== 0) throw new Error('empty URL record');
+    url['x-record'] = 'record';
+    if (JSON.stringify(Array.from(new Constructor(url))) !== '[["x-record","record"]]') {
+      throw new Error(Constructor.name + ' must use own URL properties');
+    }
+    for (const factory of [() => new Headers(), () => new URLSearchParams(), () => new FormData()]) {
+      for (const value of [undefined, null]) {
+        const input = factory();
+        let iteratorReads = 0;
+        Object.defineProperty(input, Symbol.iterator, {enumerable: true, get() {
+          iteratorReads++;
+          return value;
+        }});
+        let caught;
+        try { new Constructor(input); } catch (error) { caught = error; }
+        if (!(caught instanceof TypeError) || iteratorReads !== 1) {
+          throw new Error(Constructor.name + ' must reject the enumerable Symbol key before reading its value');
+        }
+      }
+    }
+  }
+  return 'ok';
+})()
+"#).unwrap();
+    assert_eq!(result, "ok");
+}