Performance: avoid cloning when saving document if `minimize: false`

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the issue has not already been raised

### Issue

In `$__handleSave()` we have the following:

```javascript
if (this.$isNew) {
// send entire doc
const obj = this.toObject(saveToObjectOptions);
```

However, if `minimize: false`, we don't need to run `toObject()`, we can just do `const obj = this._doc` because the MongoDB driver will clone and apply `toBSON()` anyway. This should significantly help performance.

Re: #8024, let's implement this functionality first and make sure it works as intended before considering making `minimize` default to false.

Start at $__handleSave(), inspect the $isNew branch and the existing toObject(saveToObjectOptions) path. Exercise a new document with minimize: false, compare the saved behavior, and confirm the MongoDB driver's cloning and toBSON handling preserve the document without regressions.

Written by the indexing model from the issue text.