[dwds] Fix ping event serialization by introducing PingRequest (#2821)

**Changes:**
- Replaced the raw map `{'type': 'ping'}` used for client connection checks with a proper `PingRequest` class. This fixes a `TypeError` in the browser client when trying to deserialize the raw map as a list (due to expected wire format mismatch), and aligns the ping event with the ongoing migration to standard JSON serialization in DWDS. 

**Tests:**
- Added unit tests to verify the change.

Related to #2806

Author: jyameo

dwds/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 27.1.1-wip
+
+- Replace raw map for client ping checks with a proper `PingRequest` class and update client deserialization handling.
+
 ## 27.1.0
 
 - Add `appName` to `DartDevelopmentServiceConfiguration`.

dwds/lib/data/ping_request.dart

@@ -0,0 +1,24 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// A ping request to check client connection.
+class PingRequest {
+  PingRequest();
+
+  Map<String, dynamic> toJson() => {};
+
+  factory PingRequest.fromJson(Map<String, dynamic> json) {
+    return PingRequest();
+  }
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) || other is PingRequest;
+
+  @override
+  int get hashCode => 0;
+
+  @override
+  String toString() => 'PingRequest';
+}

dwds/lib/src/handlers/dev_handler.dart

@@ -18,6 +18,7 @@ import 'package:dwds/data/hot_reload_response.dart';
 import 'package:dwds/data/hot_restart_request.dart';
 import 'package:dwds/data/hot_restart_response.dart';
 import 'package:dwds/data/isolate_events.dart';
+import 'package:dwds/data/ping_request.dart';
 import 'package:dwds/data/register_event.dart';
 import 'package:dwds/data/run_request.dart';
 import 'package:dwds/data/service_extension_request.dart';
@@ -165,7 +166,8 @@ class DevHandler {
       IsolateExit() => ['IsolateExit', request.toJson()],
       ErrorResponse() => ['ErrorResponse', request.toJson()],
       RegisterEvent() => ['RegisterEvent', request.toJson()],
-      Map() => request, // Already a raw message (e.g., ping)
+      PingRequest() => ['PingRequest', request.toJson()],
+      Map() => request,
       _ => throw UnsupportedError('Unknown request type: $request'),
     };
   }