types(models): default _id type to ObjectId for Document

[Catwallon]

Summary

This pull request changes the default generic type T for the Document class from unknown to ObjectId.

The motivation for this change is to improve the developer experience for TypeScript users by providing a more practical and intuitive default. In the vast majority of Mongoose schemas, the _id field is an ObjectId by default. Aligning the generic type T (which represents the type of _id) with this behavior reduces boilerplate and prevents potential type errors.

Currently, developers often have to explicitly define Document<ObjectId> to get proper type-safety on the _id field. By making ObjectId the default, the types will work out-of-the-box for the most common use case.

This also raises a question: I'm curious about the original rationale for choosing unknown as the default. There may be a design consideration I am not aware of, and understanding it would be valuable.

Examples

This change directly impacts type inference on document instances.

Current Behavior

With the default T = unknown, accessing methods on _id without explicit typing results in a TypeScript error.

import { Document, Schema, model } from 'mongoose';

// Interface extending `Document` uses the default generic `unknown` for the `_id`.
interface IUser extends Document {
  name: string;
}

const userSchema = new Schema<IUser>({ name: { type: String, required: true } });
const User = model<IUser>('User', userSchema);

const user = new User({ name: 'Jane Doe' });

// `user._id` is typed as `unknown`.
// The following line causes a TypeScript error:
// >> Property 'toString' does not exist on type 'unknown'.
const idString = user._id.toString();

After this PR

With the default T = ObjectId, the _id property is correctly typed, and no errors occur.

import { Document, Schema, model } from 'mongoose';

// Interface extending `Document` now uses the new default generic `ObjectId` for the `_id`.
interface IUser extends Document {
  name: string;
}

const userSchema = new Schema<IUser>({ name: { type: String, required: true } });
const User = model<IUser>('User', userSchema);

const user = new User({ name: 'Jane Doe' });

// `user._id` is correctly typed as `ObjectId`.
// This code now works perfectly without any type errors.
const idString = user._id.toString(); // OK!