Remove built_value dependency from HotRestartResponse

[jyameo]

This PR removes the package:built_value dependency from HotRestartResponse and replaces it with standard Dart JSON serialization.

Changes

• Rewrote HotReloadResponse as a simple Dart class with manual JSON serialization
• Removed HotRestartResponse from the @SerializersFor annotation in serializers.dart
• Updated client.dart to serialize HotRestartResponse using the wire format ['HotRestartResponse', response.toJson()]
• Added _deserializeMessage() helper in dev_handler.dart to deserialize both built_value and non-built_value response types
• Updated _sendHotRestartResponse in client.dart to use named parameters instead of builder pattern

Testing

• Existing tests for hot restart functionality continue to pass with the updated implementation.
• The change follows the same pattern used for HotReloadRequest, HotReloadResponse, and HotRestartRequest

Fixes dart-lang/sdk#62291