useHostStyles internals override each other

[Avcharov]

Describe the bug
useHostStyles internally uses useHostStyleVariables() and useHostFonts()

Inside useHostStyleVariables and useHostFonts are assigning different callback to app.onhostcontextchanged which causes overriding while using both of methods together inside useHostStyles:

// useHostFonts
app.onhostcontextchanged = (params) => {
  if (params.styles?.css?.fonts) {
    applyHostFonts(params.styles.css.fonts);
  }
};

// useHostStyleVariables
app.onhostcontextchanged = (params) => {
  if (params.theme) {
    applyDocumentTheme(params.theme);
  }
  if (params.styles?.variables) {
    applyHostStyleVariables(params.styles.variables);
  }
};

// useHostStyles
export function useHostStyles(
  app: App | null,
  initialContext?: McpUiHostContext | null,
): void {
  useHostStyleVariables(app, initialContext);
  useHostFonts(app, initialContext);
}

Overriding happens due to onhostcontextchanged implementation which is based on setNotificationHandler.
typescript-sdk mentions that previous handlers will be overwritten:

  /**
     * Registers a handler to invoke when this protocol object receives a notification with the given method.
     *
     * Note that this will replace any previous notification handler for the same method.
     */
    setNotificationHandler<M extends NotificationMethod>(
        method: M,
        handler: (notification: NotificationTypeMap[M]) => void | Promise<void>
    )

To Reproduce
Steps to reproduce the behavior:
Try to track host style variables changes after using useHostStyles, it wont we tracked

Expected behavior
useHostStyles() works w/o overriding onhostcontextchanged callbacks

Proposal

1. Instead of reusing useHostStyleVariables and useHostFonts inside useHostStyles just put all needed code into single callback and assign it to onhostcontextchanged.

OR

2. Draft PR: feat: implement subscribe/unsubscribe methods for multiple event handlers #552
   Implement subscribe(eventName) / unsubscribe(eventName) methods for multiple handlers. Then useHostStyleVariables and useHostFonts would look like:

//useHostFonts
const unsubscribe = app.subscribe("hostcontextchanged", (params) => {
  if (params.styles?.css?.fonts) {
    applyHostFonts(params.styles.css.fonts);
  }
});

return unsubscribe;

//useHostStyleVariables
const unsubscribe = app.subscribe("hostcontextchanged", (params) => {
  if (params.theme) {
    applyDocumentTheme(params.theme);
  }
  if (params.styles?.variables) {
    applyHostStyleVariables(params.styles.variables);
  }
});

return unsubscribe;

[Avcharov]

@ochafik Can you kindly take a quick look?