[Feature]: Register plugins with Rivet core chat-related facilities

### Feature Request

Current implementation of Rivet assumes that only nodes of type 'chat' and to some extent 'chatAntropic' are integrated in various editor components, such as `ChatViewer`.
I'd suggest adding an extension point allowing for plugins to register for such services.
I'm also willing to implement it.

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Start by tracing how nodes of type `chat` and `chatAntropic` are integrated into editor components such as `ChatViewer`. Identify the existing Rivet core chat-related facilities and determine the extension-point design needed for plugins to register with them. Done means plugins can register for those services without relying on built-in node types; no specific test file is named.

Written by the indexing model from the issue text.