claude: fix column-margin with renderings option (#13825)

Preserve existing classes on cell-output-display divs when applying
light/dark theme classes in the cell-renderings filter. Previously,
new divs were created that discarded existing classes like column-margin.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gordonwoodhull

src/resources/filters/quarto-post/cell-renderings.lua

@@ -57,8 +57,11 @@ function choose_cell_renderings()
           blocks:insert(darkDiv)
         end
       elseif quarto.format.isHtmlOutput() and lightDiv and darkDiv then
-        blocks:insert(pandoc.Div(lightDiv.content, pandoc.Attr("", {'light-content'}, {})))
-        blocks:insert(pandoc.Div(darkDiv.content, pandoc.Attr("", {'dark-content'}, {})))
+        -- Preserve existing classes (e.g., column-margin, cell-output-display) and add theme class
+        lightDiv.classes:insert('light-content')
+        darkDiv.classes:insert('dark-content')
+        blocks:insert(lightDiv)
+        blocks:insert(darkDiv)
       else
         blocks:insert(lightDiv or darkDiv)
       end

tests/docs/smoke-all/dark-mode/renderings-column-margin.qmd

@@ -0,0 +1,27 @@
+---
+title: "renderings with column-margin"
+format:
+  html:
+    theme:
+      light: flatly
+      dark: darkly
+_quarto:
+  tests:
+    html:
+      ensureHtmlElements:
+        -
+          - 'div.cell div.column-margin div.cell-output-display.light-content'
+          - 'div.cell div.column-margin div.cell-output-display.dark-content'
+        - []
+---
+
+This tests that `column: margin` works together with `renderings: [light, dark]`.
+The column-margin class should be preserved when creating light/dark content divs.
+
+```{r}
+#| column: margin
+#| renderings: [light, dark]
+
+knitr::kable(data.frame(x = "Light mode", y = 1))
+knitr::kable(data.frame(x = "Dark mode", y = 2))
+```