WPT script-transform-generateKeyFrame.https.html is not aligned with specification

[youennf]

Similarly to #243, it seems the spec and WPT test are not in sync.
The WPT test expects that transformer.generateKeyFrame reject with InvalidStateError when the sender is negotiated inactive or when the sender has no track.
InvalidStateError can only be used synchronously so it is probably related to the line:
If encoder is undefined, reject promise with [InvalidStateError](https://webidl.spec.whatwg.org/#invalidstateerror), abort these steps.

encoder is not set to undefined when sender track is null or in case of inactive state.

We should probably update the test, which is part of WPT interop 2025.
@guidou, @jan-ivar, thoughts?

[jan-ivar]

generateKeyFrame and sendKeyFrameRequest both suffered the same prose ambiguity we resolved in #230 (comment) so the same decision seems to apply.

I see the WPT tests for both were updated in concert in https://bugs.webkit.org/show_bug.cgi?id=296559, so what remains is a PR to fix the generateKeyFrame prose similar to #247.