- Updated `LegacyContext.send_message` to handle rich message chains using `send_chain`.
- Introduced `LegacyStar` class for backward compatibility with legacy plugins.
- Added `register` decorator for legacy plugin metadata.
- Enhanced `MessageChain` class with `to_payload` and `is_plain_text_only` methods.
- Updated `AstrMessageEvent.send` method to utilize `send_chain` for rich messages.
- Implemented `send_chain` method in `PlatformClient` for sending complex message structures.
- Added capability routing for `platform.send_chain`.
- Introduced tests for new functionality, ensuring compatibility with legacy plugins and message handling.
- Implement tests for HandlerDispatcher in `tests_v4/test_handler_dispatcher.py`, covering initialization, invocation, cancellation, argument building, result consumption, error handling, and handler execution.
- Introduce tests for Plugin loading functionalities in `tests_v4/test_loader.py`, including plugin specification, discovery, environment management, and loading components and handlers.
