AutoConvert Issue: String Field Accepts Object Containing a String

Nobody has claimed this yet.

**Describe the bug**

Hey Simple-schema team,
First off, thanks for all your amazing work! I've run into a confusing situation and would appreciate some clarification. When using a string field, it seems that passing an object that contains a string (e.g., `{ foo: 'bar' }`) still passes validation. Did I misunderstand how `autoConvert` works, or is this behavior a bug?

**To Reproduce**

- Set `autoConvert` to `true` (or leave it at its default setting)
- Define a field with type `String`
- Pass an object containing a string to that field, for example:
```js
{ 'stripeInvoice.invoiceUrl': { foo: 'bar' } }
```
- Notice that the validation passes unexpectedly

**Expected behavior**

I expected the validation to fail when an object is provided, even if it contains a string. I understand that `autoConvert` performs some conversions for string fields by default when not explicitly disabled, but I assumed that objects with string values would not be automatically converted. Clarification on this behavior would be appreciated.

**Screenshots**


**Versions (please complete the following information):**
- Meteor version: [2.16]
- Browser: [Chrome]
- Version: [1.13.1] (I know we haven't been able to upgrade to the latest version, but we will be soon)

**Additional context**

This is my first ever question on a GitHub repo. Please let me know if any further details are needed. Thanks again for your support! 🙏

Start by reproducing the reported case with autoConvert enabled, a String field, and the `{ 'stripeInvoice.invoiceUrl': { foo: 'bar' } }` value. Review the autoConvert behavior and existing validation expectations; the issue does not name a source file or test, and done would require deciding whether this conversion should be rejected or clarified.

Written by the indexing model from the issue text.