`ValidationError: CastError: TypeError: path.split is not a function` with `refPath` as a function

### 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

9.2.1

### Node.js version

25.4.0

### MongoDB server version

not relevant

### Typescript version (if applicable)

5.9.3

### Description

Trying to use `refPath` with a function and assign a document to that property in `create`, it results in a undescriptive `ValidationError`:
```txt
ValidationError: RefPathTest validation failed: ref: Cast to ObjectId failed for value "{
  propA: 'helloA',
  _id: new ObjectId('699069acf888d8b513312cd9'),
  __v: 0
}" (type model) at path "ref" because of "TypeError"
```

Upon further investigation, this is caused by a `TypeError` inside mongoose:
```txt
reason: TypeError: path.split is not a function
    at Schema.hasMixedParent (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/schema.js:1944:25)
    at Schema.path (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/schema.js:1288:23)
    at model.Document.$__path (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:2034:25)
    at model.Document.get (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1943:65)
    at /mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1435:29
    at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1437:7)
    at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1170:16)
    at model.Document (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:181:12)
    at model.Model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:131:12)
    at new model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:4805:15)
    at /mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2747:20
    at Array.map (<anonymous>)
    at model.create (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2735:34)
    at Object.<anonymous> (/mnt/ssd/projects/nodejs/typegoose/test/tests/ref.test.ts:484:40)
    at processTicksAndRejections (node:internal/process/task_queues:104:5),
```

I had also tried with other property names in case they were messing with "special properties", but the error remained the same.

Finally, note that assigning a document to a `refPath` property that is not a function works as expected (where it gets cast to the reftype, in this case `ObjectId`)

<details>
<summary>Full Error</summary>

```txt
ValidationError: RefPathTest validation failed: ref: Cast to ObjectId failed for value "{
  propA: 'helloA',
  _id: new ObjectId('699069acf888d8b513312cd9'),
  __v: 0
}" (type model) at path "ref" because of "TypeError"
    at model.Document.invalidate (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:3405:32)
    at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1522:12)
    at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1170:16)
    at model.Document (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:181:12)
    at model.Model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:131:12)
    at new model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:4805:15)
    at /mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2747:20
    at Array.map (<anonymous>)
    at model.create (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2735:34)
    at Object.<anonymous> (/mnt/ssd/projects/nodejs/typegoose/test/tests/ref.test.ts:484:40)
    at processTicksAndRejections (node:internal/process/task_queues:104:5) {
  errors: {
    ref: CastError: Cast to ObjectId failed for value "{
      propA: 'helloA',
      _id: new ObjectId('699069acf888d8b513312cd9'),
      __v: 0
    }" (type model) at path "ref" because of "TypeError"
        at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1523:9)
        at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1170:16)
        at model.Document (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:181:12)
        at model.Model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:131:12)
        at new model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:4805:15)
        at /mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2747:20
        at Array.map (<anonymous>)
        at model.create (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2735:34)
        at Object.<anonymous> (/mnt/ssd/projects/nodejs/typegoose/test/tests/ref.test.ts:484:40)
        at processTicksAndRejections (node:internal/process/task_queues:104:5) {
      stringValue: '"{\n' +
        "  propA: 'helloA',\n" +
        "  _id: new ObjectId('699069acf888d8b513312cd9'),\n" +
        '  __v: 0\n' +
        '}"',
      messageFormat: undefined,
      kind: 'ObjectId',
      value: [Object],
      path: 'ref',
      reason: TypeError: path.split is not a function
          at Schema.hasMixedParent (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/schema.js:1944:25)
          at Schema.path (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/schema.js:1288:23)
          at model.Document.$__path (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:2034:25)
          at model.Document.get (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1943:65)
          at /mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1435:29
          at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1437:7)
          at model.$set (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:1170:16)
          at model.Document (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/document.js:181:12)
          at model.Model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:131:12)
          at new model (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:4805:15)
          at /mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2747:20
          at Array.map (<anonymous>)
          at model.create (/mnt/ssd/projects/nodejs/typegoose/node_modules/mongoose/lib/model.js:2735:34)
          at Object.<anonymous> (/mnt/ssd/projects/nodejs/typegoose/test/tests/ref.test.ts:484:40)
          at processTicksAndRejections (node:internal/process/task_queues:104:5),
      valueType: 'model'
    }
  },
  _message: 'RefPathTest validation failed'
}

```

</details>

### Steps to Reproduce

```ts
const childASchema = new Schema({ propA: { required: true, type: String } });
const childBSchema = new Schema({ propB: { required: true, type: String } });
const ChildAModel = mongoose.model('ChildA', childASchema);
const ChildBModel = mongoose.model('ChildB', childBSchema);

const indexes = [ChildAModel.modelName, ChildBModel.modelName];

const refPathTestSchema = new mongoose.Schema({
  model: { required: true, min: 0, max: 1, type: Number },
  ref: {
    refPath: function () {
      return indexes[this.model];
    },
    type: mongoose.Types.ObjectId,
  },
});
const RefPathTestModel = mongoose.model('RefPathTest', refPathTestSchema);

const childAdoc = await ChildAModel.create({ propA: 'helloA' });
await ChildBModel.create({ propB: 'helloB' });

try {
  const doc = await RefPathTestModel.create({ model: 0, ref: childAdoc } as any);
} catch (err) {
  // necessary, as otherwise you dont get the actual error printed
  console.error(err);
  throw err;
}
```

### Expected Behavior

No error, or a more descriptive error on what went wrong.

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Uh oh!

`ValidationError: CastError: TypeError: path.split is not a function` with `refPath` as a function #16028

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Typescript version (if applicable)

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Uh oh!

ValidationError: CastError: TypeError: path.split is not a function with refPath as a function #16028

Description

Prerequisites

Mongoose version

Node.js version

MongoDB server version

Typescript version (if applicable)

Description

Steps to Reproduce

Expected Behavior

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

`ValidationError: CastError: TypeError: path.split is not a function` with `refPath` as a function #16028