[python] Ser/deserialization for complex union cases have issues

transferred from https://github.com/Azure/autorest.python/issues/3006

This is derived from [discussion ](https://github.com/msyyc/typespec-e2e-demo/pull/2#discussion_r1889565930)e2e demo, the current ser/deserialization logic has issues to handle the complex union cases like below.

After confirmed with @msyyc we know that the current implementation would fail if our union contains more than one models. Or may also fail in other cases.

In this issue we may need to decide either to accept the current situation or re-design for un-supported cases.

```
@jsonSchema
model TodoFileBytesAttachment extends TodoAttachment {
kind: TodoAttachmentType.File;
/** The file contents */
file: bytes;
}

@jsonSchema
model TodoUrlAttachment extends TodoAttachment {
kind: TodoAttachmentType.Url;
/** A description of the URL */
description: string;

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

@jsonSchema
model TodoAttachment {
/** discriminator kind */
kind: TodoAttachmentType;
}

@jsonSchema
union TodoAttachmentType {
string,
File: "file",
Url: "url",
}
```