`Plot::show()` panics when no opener binary exists, and silently no-ops when it fails

Plot::show() offers no way for a caller to handle a failed browser handoff.
Running it on a headless server causes a panic, but the panic message is misleading.

How to reproduce

use plotly::{Plot, Scatter};

fn main() {
    let mut plot = Plot::new();
    plot.add_trace(Scatter::new(vec![0, 1, 2], vec![0.0, 1.5, 3.0]));
    plot.show();
}

Run on a headless Linux host with no xdg-open on PATH 👍

thread 'main' panicked at .../plotly-0.14.1/src/plot.rs:789:14:
Could not find default application for HTML files.
[...]
: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Environment

• plotly 0.14.1
• rustc 1.98.0
• AlmaLinux 9.8

Expected

A library call should not abort the process over a missing external binary. The caller should have
some way to detect this and potentially fall back to write_html() or write_image() on its own or
show a clear error message.

Cause

https://github.com/plotly/plotly.rs/blob/00fe0512e9f924bd0fc2edfc6cc617a1604f4e0d/plotly/src/plot.rs#L793-L800

There are two problems with it:

1. Missing binary panics: .output() returns Err only when the process cannot be
   spawned, i.e. xdg-open is not on PATH. .expect() turns that into a panic, and
   show() returns (), so a caller has nothing to match on.
   The resulting error message is very misleading here: https://github.com/plotly/plotly.rs/blob/00fe0512e9f924bd0fc2edfc6cc617a1604f4e0d/plotly/src/plot.rs#L57-L65

2. Return code of the xdg-open is ignored: If xdg-open is installed but exits non-zero
   (e.g. when no application is registered for HTML) it still returns Ok, carrying
   the failure inside Output::status, but status is never read.

The macOS and Windows variants have a similar shape.

Commencez dans plotly/src/plot.rs, au niveau de l’implémentation référencée de Plot::show(), et examinez les branches Linux, macOS et Windows, y compris les définitions d’erreurs vers les lignes 57-65. Reproduisez le cas Linux headless, puis vérifiez que les binaires opener manquants et les statuts opener non nuls sont signalés aux appelants au lieu de provoquer un panic ou d’être ignorés silencieusement.