4.0 htmx.ajax adds form fields to the URL if the target is a form

Showcase:

```html

htmx.config.logAll = true;

const url = new URL(window.location.href);
url.searchParams.set("some-param", "some-value");
htmx.ajax('GET', url.toString(), {
target: 'form'
});

```

Results in a request to `/index.html?some-param=some-value&some-other-param=some-other-value` instead of the expected `/index.html?some-param=some-value`

This was not the case in htmx 2.x.

Update:

The problem seems that the fallback to `source` ends up being the `target` while the expected behavior is this:

> With no source or target, htmx uses document.body.

https://four.htmx.org/reference/methods/htmx-ajax

Workaround: add `source: document.body` to the `htmx.ajax` options.

Start with the htmx.ajax entry point and reproduce the showcase using a form target and a GET URL with query parameters. Trace how source and target are chosen when source is omitted; done means the request URL contains the original query parameter but not fields from the target form, matching htmx 2.x behavior.

Written by the indexing model from the issue text.