MarkdownToNotebook: captured kernel messages render as a plain text cell, not the styled MessageTemplate2 banner a live evaluation produces

[mbahram]

Summary

When MarkdownToNotebook[..., "Evaluate" -> True] captures a message emitted by an executable cell, messageCell[s_String] wraps the captured text in a bare Cell[s, "Message", "MSG"] — plain, unstyled text. An interactive front end renders the same message as a TemplateBox[{...}, "MessageTemplate2"] styled banner, carrying {symbol, tag, formatted-text, 2, 1, 2, id, "Local", full-name} and prefixed with a During evaluation of In[n]:= cell label.

As a result, captured messages in a built notebook do not look like what the same code produces when evaluated live: no styled banner, no "During evaluation of" label, no message-suppression affordance.

Repro

# T

```wl #| eval: true
Sqrt[x] /. x -> {1, 2, 3}

A cell that throws (e.g. a `::tag` message) captures the message text and currently emits it as a plain `Cell["Sym::tag: body", "Message", "MSG"]`.

## Expected

The captured `Sym::tag: body` text is reconstructed as the front end's `MessageTemplate2` `TemplateBox`, so the built cell matches a live evaluation (styled banner + `During evaluation of In[1]:=` label). A string that is **not** in `Symbol::tag` form should fall back to the plain text `Message` cell.

## Notes (root cause)

`messageCell[s_String]` in `MarkdownToNotebook.wl`. The kernel only ever writes a message as text; the styled box is an FE rendering. Reconstruct it by parsing `Sym::tag: body` and emitting the `MessageTemplate2` box. The `id` / context args drive only the suppression menu, not the display (rendering is identical for any id/context), so a best-effort `Names[sym]` is sufficient; the formatted body must be a quoted string box (`ToString[.., InputForm]`) or the FE parses a `-` in the text as a math operator.

[sw1sh]

Considered — keeping the current form for now rather than reconstructing the MessageTemplate2 banner, which is fragile:

• I couldn't obtain a reliable MessageTemplate2 box reference even from a headless front end, so the 9-arg structure would have to be reconstructed blind.
• The template renders as raw template args if the page stylesheet doesn't define it.
• The "During evaluation of In[n]:=" cell label would be fabricated (captured batch messages have no real In[n]).
• The new box shape risks the NotebookToMarkdown round-trip, which currently reads the Message cell cleanly.

The existing Cell[BoxData[RowBox[{StyleBox[name, "MessageName"], ": ", body}]], "Message", "MSG"] is already a faithful, round-trippable representation (message-styled cell + styled Sym::tag name). Leaving this open pending a real MessageTemplate2 reference from a live desktop FE — happy to revisit then.