Fix keyboard activation of the voice input mode microphone button

[Copilot]

Description

The segmented voice input mode control (dictation / voice / listen cells) could not be started or stopped with the keyboard: focusing a cell and pressing Enter/Space did nothing.

Root cause: the cells are <button> elements inside a toolbar's ActionBar. The ActionBar's KEY_DOWN handler intercepts Enter/Space, runs the item's action — the no-op placeholder ChatVoiceInputModeAction.run() — and calls preventDefault()/stopPropagation(), swallowing the native button activation before the cell's own gesture can run. (This also produced the two focus stops testers observed: the ActionBar action-item and the inner button.)

Change (voiceInputModeActionViewItem.ts):

• Added _registerActivationKeys(cell, handler) — an Enter/Space KEY_DOWN listener that stops the event before it reaches the ActionBar and invokes the same handler as the click gesture.
• Wired it to all three cells: dictation → _onClickDictation, voice → _onClickVoicePowerToggle, listen → _onClickListen.

private _registerActivationKeys(cell: HTMLElement, handler: () => void): void {
    this._register(dom.addStandardDisposableListener(cell, dom.EventType.KEY_DOWN, e => {
        if (e.equals(KeyCode.Enter) || e.equals(KeyCode.Space)) {
            e.preventDefault();
            e.stopPropagation();
            handler();
        }
    }));
}

Enter/Space now activate the focused cell, complementing the existing keybindings. Mouse/pointer and hold-to-talk behavior are unchanged.

• Fixes Fancy animated button cannot be stopped by keyboard alone #328756