amp-form verify XHR seems broken with enctype=application/x-www-form-urlencoded

### Description

amp-form verify XHRs seem broken when the form has `enctype=application/x-www-form-urlencoded`.

/to @krdwan
/to @samouri

### Reproduction Steps

Repro AMP HTML:

```

That username is already taken
Invalid character in username

No verify XHR is sent after each change to the input fields. Removing `enctype=application/x-www-form-urlencoded` fixes the issue.

It looks like the bug was introduced in #27644. The following line conditional sets `body` to a string:

https://github.com/ampproject/amphtml/blob/f564b7ee3cd59972fe86c4edc3052db70852c6f0/extensions/amp-form/0.1/amp-form.js#L313-L314

However, a few lines down the code unconditionally assume `body` is a `FormDataWrapper` object and calls `append` on it:

https://github.com/ampproject/amphtml/blob/f564b7ee3cd59972fe86c4edc3052db70852c6f0/extensions/amp-form/0.1/amp-form.js#L328-L330

### Relevant Logs

_No response_

### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start in extensions/amp-form/0.1/amp-form.js around the linked lines that construct the verify XHR body and append form data. Reproduce the AMP form example with enctype="application/x-www-form-urlencoded", then verify that changes trigger the verify XHR and that existing form behavior remains intact.

Written by the indexing model from the issue text.