Mongoose 8.7.2 breaks Union Types

[timheerwagen]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.7.2

Node.js version

22

MongoDB server version

6.11

Typescript version (if applicable)

5.7.2

Description

Before 8.7.2, only the type of the union was returned and that was fine. But now the type is returned as binary, but the union has no binaries in it.
It does not matter that the correct union value is not parsed here by mongoose, as this is done by zod.
This error seems to be related to this issue: #14974

Steps to Reproduce

import mongoose, { Model } from "mongoose";

type Attachment =
  | {
      type: "foo";
      value?: undefined;
    }
  | {
      type: "string";
      value?: string;
    };

const TestSchema = new mongoose.Schema<Attachment>({
  type: { type: String, required: true },
  value: { type: String },
});

export const AttachmentModel = mongoose.model<Attachment>("test", TestSchema);

const main = async () => {
  const item = await AttachmentModel.findOne().lean();

  if (!item) return;

  doSomeThing(item);
};

const doSomeThing = (item: Attachment) => {
  console.log(item);
};

doSomeThing returns error

Expected Behavior

No response