fix: set openWhenHidden to be true

[yanCode]

deep in the source code of fetch-event-source , (not the readme markdown document). you can find something here:

    /**
     * If true, will keep the request open even if the document is hidden.
     * By default, fetchEventSource will close the request and reopen it
     * automatically when the document becomes visible again.
     */
    openWhenHidden?: boolean;

this is much like TanStack Query or SWR
which stops http fetching while the browser tab is hidden (shift to be inactive, to back). and restart to fetch data when current tab is shifted back. I personally believe this is a great feature for most scenarios fetching short responses to do cache and revalidate cache. but not the case for a long streaming response.. so the solution to it is to set openWhenHidden: true, this makes it continue loading long contents without interruptions even if current tab is shifted back and forth.

This PR fixes the bug reported in #1566

[Yidadaa]

Great work, this really saves my life 😆