Clarify the scope of supported cases in todo spec in e2e demo

[MaryGao]

Below two cases are not supported yet in some emitters in @markcowl's todo spec pr. I'd like to clarify if both of them are in Jan e2e demo's scope.

• Unsupported 1: Model is shared among mfd and non-mfd operations

TodoAttachment will be shared in a json operation createJson(link) and a mfd operation createForm(link). Since currently emitters would treat mfd model differently so sharing here would be an issue for Java and JavaScript and I didn't check other language yet.

I am also clarifying if there is any limitation for these cases: microsoft/typespec#5376.

• Unsupported 2: Union cases with bytes type

I know other lanagues would only generate protocol methods for this. This seems to be a JS issue only, currently we can't handle the deserialization/serialization for union model TodoAttachment without a discriminator property.

Full support of union deserialization/serialization are problematic and a big issue in JS. Not sure we could scope to discriminated union for now?

union TodoAttachment {
  file: TodoFileAttachment,
  url: TodoUrlAttachment,
}

model TodoFileAttachment is File;

model File {
  contentType?: string;
  filename?: string;
  contents: bytes;
}

model TodoUrlAttachment {
  /** A description of the URL */
  description: string;

  /** The url */
  url: url;
}

[lirenhe]

cc @lmazuel @joheredi for awareness.

[lmazuel]

As we discussed, let's make the TypeSpec what we support, and keep this discussion for Jan/Feb 2025