Fix `NotFoundSetOnFormSubmit_ResponseNotStarted_SSR` test

[ilonatommy]

NotFoundSetOnFormSubmit_ResponseNotStarted_SSR should use "post-not-found-ssr" url, not "post-not-found-ssr-streaming". After fixing this test, we uncovered an issue with fetching 404 page by the client.

It turns out that in the cases when the NotFound was called synchronously, it was reaching the enhanced navigation code on client earlier than initialization of enhanced navigation in boot function. To fix this we did 2 things:

• moved enhanced nav initialization as early as possible in boot (just a cosmetic change),
• changed navigationEnhancementCallbacks to a promise, so that we make sure it's not undefined when we enter performEnhancedPageLoad.

To do it, take #62524 and separate connected changes into a PR.

[javiercn]

@ilonatommy Is this fixed as part of #62635?

Closing for now, but feel free to reopen