Fix repeated cancellation message on Ctrl-C during API key prompt

[Copilot]

Pressing Ctrl-C during the "Enter API Key" prompt triggers the OnCancelKeyPress handler repeatedly while the blocking prompt waits for input, flooding the console with cancellation messages.

Fix: Guard against re-entry in OnCancelKeyPress:

private static void OnCancelKeyPress(object? sender, ConsoleCancelEventArgs e)
{
    e.Cancel = true;

    if (_wasCancelled)
        return;

    _wasCancelled = true;
    _cts?.Cancel();
    AnsiConsole.MarkupLine("[yellow]Cancellation requested. Shutting down gracefully...[/]");
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ctrl-C at "Enter API Key" hard-locks the graceful shutdown</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

@refringe Thanks for reporting this.

• Fixes Ctrl-C at "Enter API Key" hard-locks the graceful shutdown #21

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