Terminate Quarto preview when terminal is closed externally

[juliasilge]

Addresses posit-dev/positron#13006.

When a user closes the Quarto Preview terminal via the trash icon, the underlying Quarto process was not terminated cleanly. This left behind intermediate .quarto_ipynb files, and repeated previews would accumulate *.quarto_ipynb_1, *.quarto_ipynb_2, etc.

This was happening because VS Code's trash icon sends SIGHUP to the shell process only. Quarto, as a child process, typically survives as an orphan and never receives the signal needed to trigger cleanup. Ctrl+C works because it sends SIGINT to the entire foreground process group.

This PR registers an onDidCloseTerminal listener in PreviewManager. When the preview terminal is closed by any external means, it sends the same HTTP termination request that killPreview() already uses, giving Quarto the chance to clean up before it exits. To avoid a duplicate request when the extension itself kills the terminal, this.terminal_ is cleared before killTerminal() is called, so the event handler's identity check short-circuits.

[vezwork]

This change looks good to me. Thank you for the clear explanation of the problem and solution.

I didn't test this PR locally, as I understand it will be verified that the issue was resolved separately.