fix: allow multiple bindCLIShortcuts calls with shortcut merging

[edmundhung]

Fixes #15781.
Closes #15881.

This fixes an issue where calling bindCLIShortcuts() multiple times would create duplicate readline instances and lose custom shortcuts from previous calls. This is useful for plugins adding their own shortcuts while Vite CLI continues to print the help message.

Now when you bind shortcuts multiple times, they are properly merged, with new custom shortcuts taking priority:

server.bindCLIShortcuts({
  customShortcuts: [
    { key: 's', description: 'My shortcut', action: () => console.log('foo') },
    { key: 'g', description: 'Greeting', action: () => console.log('Hello World') },
  ]
});

// The "enter + g" shortcut will remain available after binding
server.bindCLIShortcuts({
  customShortcuts: [
    // This overrides the previous "enter + s" shortcut
    { key: 's', description: 'my updated shortcut', action: () => console.log('foobar') },
  ]
});

This works for both dev and preview server.

[sapphi-red]

Thank you!

[ArnaudBarre]

Looks good thanks!

[ArnaudBarre]

It's probably a miss-understanding of the mocking system but I don't understand this assertion. Does using toHaveBeenCalledOnce above "reset" the counter? Otherwise I would have use toHaveBeenCalledOnce again

[edmundhung]

I reset the counter with xAction.mockClear() right before calling bindCLIShortcuts again. If you find it confusing, happy to remove that and just assert whether xAction is still only called once.

[ArnaudBarre]

Oh I missed that line. Ok makes more sense thanks. I think it just taste then so no need to change

[edmundhung]

Thanks for the review, @sapphi-red and @ArnaudBarre 🙌🏼