`toObject` does not keep populated types

### Prerequisites - [x] I have written a descriptive issue title - [x] I have searched existing issues to ensure the bug has not already been reported ### Mongoose version 9.2.2 ### Node.js version 24.13.1 ### MongoDB server version n/a ### Typescript version (if applicable) 5.8.3 ### Description calling `toObject` after `populate` does not keep the populated type in the plain object type ### Steps to Reproduce ```ts const childSchema = new Schema({ name: String }); const Child = mongoose.model('Child', childSchema); const parentSchema = new Schema({ title: String, children: [{ type: Schema.Types.ObjectId, ref: 'Child' }] }); const Parent = mongoose.model('Parent', parentSchema); async function reproduce() { const parentDoc = new Parent({ title: 'My Parent', children: [] }); const populatedDoc = await parentDoc.populate<{ children: Types.DocumentArray<HydratedDocFromModel<typeof Child>>; }>('children'); const childNameFromHydrated = populatedDoc.children[0]?.name; // OK const plainObject = populatedDoc.toObject(); const childNameFromPlain = plainObject.children[0].name; // Property 'name' does not exist on type 'ObjectId'. ts(2339) } ``` ### Expected Behavior _No response_