test(plot): pin standard backend in plot communication test

Now that the build installs Suggests (dependencies=TRUE), jgd is present
on CI runners. The plot test left r.plot.backend unset, so resolveBackend
returned 'auto', which prefers jgd when installed — the r.standardPlot
webview was never created and the spy timed out.

Pin r.plot.backend to 'standard' in the test's config stub so the test
exercises a deterministic backend regardless of which optional plot
packages are installed.

Author: grantmcdermott

src/test/suite/session.test.ts

@@ -118,6 +118,11 @@ suite('Session Communication', () => {
                 if (key === 'sessionWatcher') { return true; }
                 if (key === 'rterm.option') { return ['--no-save']; }
                 if (key === 'plot.useHttpgd') { return false; }
+                // Pin the standard plot backend so this test is deterministic.
+                // Otherwise it resolves to 'auto', which prefers jgd when jgd is
+                // installed (now the case in CI since the build installs Suggests),
+                // and the r.standardPlot webview is never created.
+                if (key === 'plot.backend') { return 'standard'; }
                 if (key === 'session.data.pageSize') { return 500; }
                 if (key === 'session.viewers.viewColumn') { return {
                     plot: 'Two',