Add plain.qmd test fixture for manual preview tests

cderv · cderv · commit 468f554fc333 · 2026-04-02T16:04:32.000+02:00
Required by T9/T17/T18 in the manual preview test matrix.
This fixture exists on main but was not part of the backport.
diff --git a/tests/docs/manual/preview/plain.qmd b/tests/docs/manual/preview/plain.qmd
@@ -0,0 +1,9 @@
+---
+title: "Plain QMD Test"
+---
+
+## T9: No code cells
+
+No .quarto_ipynb should ever be created.
+
+Edit counter: 0
