DecoratedCodeBlock has no docx/pptx renderer — code chunk `filename` header silently dropped

Summary

A code block decorated with a filename (e.g. {r filename="hello.R"} or a .filename attribute) renders correctly in HTML, LaTeX, and Markdown output, but the filename header is silently dropped for docx and pptx (and any other format with no dedicated renderer registered).

Where the gap is

src/resources/filters/customnodes/decoratedcodeblock.lua registers exactly four renderers for the DecoratedCodeBlock custom AST node:

1. A default/fallback renderer (predicate always true) — walks the code block for folding only:

   _quarto.ast.add_renderer("DecoratedCodeBlock",
     function(_) return true end,
     function(node)
       return _quarto.ast.walk(node.code_block, { CodeBlock = render_folded_block })
     end)
   

2. A markdown renderer (_quarto.format.isMarkdownOutput())
3. A latex renderer (_quarto.format.isLatexOutput())
4. An html renderer (_quarto.format.isHtmlOutput())

Only the html and latex renderers actually read node.filename and render a header for it. Since docx/pptx match none of the three specific predicates, they fall through to the default renderer, which never reads node.filename (or node.caption) at all — it only walks for render_folded_block (itself a no-op for non-HTML output). The result: the code block content survives, but the filename header silently disappears, with no warning.

Repro

---
title: repro
---

```{.python filename="hello.py"}
print("hello")

Render with `quarto render repro.qmd --to docx` (or `--to pptx`). The code block appears; the "hello.py" filename header does not. Compare with `--to html` or `--to latex`, both of which show it.

## Suggested fix

Add a docx/pptx-aware renderer for `DecoratedCodeBlock` (or extend the default renderer to check `_quarto.format.isDocxOutput()`/`isPowerPointOutput()`) that emits *some* visible representation of `node.filename` — e.g. a bold paragraph immediately before the code block, mirroring the semantic intent of the html wrapper (`code-with-filename`) and the latex `\caption`, adapted to what Pandoc's docx/pptx writers can represent natively (Word/PowerPoint have no custom CSS to hook into, so this would need to be a plain paragraph/text run rather than a styled wrapper).

## Context

Found while auditing `decoratedcodeblock.lua`'s renderer coverage against Pandoc's non-core output targets (docx/pptx) for a project that reuses these filters. Confirmed against a fresh clone at `v1.11.3` (and current `main`, `83d48d8e8` — the file is unchanged between the two).

Start with src/resources/filters/customnodes/decoratedcodeblock.lua and compare the existing HTML and LaTeX renderers with the fallback path. Render the provided repro with --to docx and --to pptx, then verify that a visible filename representation appears before the code block while HTML and LaTeX behavior remains unchanged.