docs(ct): document class instance serialization limitation

[furkankoykiran]

Summary

• Document that class instances lose their prototype methods when passed as mount() props, due to Node-to-browser serialization

Fixes #22194

[Skn0tt]

Thanks for the PR! I worry that the warning will produce a lot of false-positive noise, e.g. for users that make frequent use of DTO-style classes.

class UserDTO {
  constructor(readonly name: string) {}
}
await mount(<Button title="test" data-model={new UserDTO("Alice")} />);

We don't currently have a good way of exposing a warning just for failing tests. Maybe let's just do the docs for now?

[furkankoykiran]

Makes sense -- a DTO that just carries data isn't broken by serialization, so warning on every class instance would produce a lot of noise for no real benefit. Removed the runtime warning and its test, kept the docs update, and tightened the example per your suggestion (dropped the separate code block in favor of the inline comment in the existing "this will not work" block).

[Skn0tt]

Nice!