fix(embed): don't use a non-leading heading as embedded notebook title

[cderv]

TL;DR

An embedded notebook with no title and no heading cell used a code-cell comment (e.g. # plt.savefig(...)) as its "Source" title. This PR gates heading-to-title promotion in findTitle on "nothing precedes the heading" (option A), so a # inside a code fence can't be the title and the filename is used instead. A side effect: a real markdown heading with prose before it also falls back to the filename — intentional, because a standalone notebook's own title already behaves that way. A more surgical fence-aware fix (option B) is described below but not taken here.

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When embedding a notebook that has no YAML title and no markdown heading cell, the "Source" link and sidebar showed a Python code comment (e.g. # plt.savefig(...)) instead of the notebook filename.

Root Cause

findTitle() in src/core/jupyter/jupyter-embed.ts scans every cell's markdown to find a title. A code cell's markdown is its fenced source, and partitionMarkdown is not fence-aware, so a # comment line inside the fence matches the ATX-heading regex and gets returned as the title.

The non-embed path already guards against exactly this. The Dec-2023 "Improve title snipping in notebooks" change (24672a4, #5363/#6411) made fixupFrontMatter only promote a heading to the notebook title when no content precedes it. findTitle predates that work and was never brought in line, so it still promoted any heading-like line.

Fix (option A)

Expose contentBeforeHeading (already computed by markdownWithExtractedHeading) through PartitionedMarkdown, and gate findTitle on !partitioned.contentBeforeHeading. A fenced code block's opening delimiter always counts as content before the comment, so the comment is no longer mistaken for a title and the filename is used instead.

This borrows the same content-before-heading gate that fixupFrontMatter uses. The two functions are not otherwise equivalent — findTitle scans every cell and can't filter on cell_type, whereas fixupFrontMatter inspects only the first markdown cell.

Behavior note

Because the gate keys off "content before the heading" rather than "is this a code cell", it also affects a real markdown cell whose heading has prose before it: that heading is no longer promoted, and the embed "Source" title falls back to the filename.

This is intentional and is the reason to prefer option A. A standalone notebook with prose before its first heading already gets no title from that heading — fixupFrontMatter only promotes a heading when nothing precedes it (since #5363 / #6411). Option A makes the embed "Source" title behave the same way as the notebook's own title, instead of using a heading that the rest of Quarto would not treat as the document title.

The documented precedence is YAML title, then the first markdown heading, then the filename. Option A is marginally stricter than the literal "first markdown heading" wording for the prose-before-heading case, but matches the actual title behavior of a standalone notebook.

Alternative (option B, not taken here)

The root cause mcanouil identified is that heading extraction is not fence-aware. Option B would make markdownWithExtractedHeading (or findTitle) skip # lines inside fenced code, fixing only the code-comment case and leaving a prose-before-heading markdown cell promotable — matching the literal documented "first markdown heading" wording.

Option B is the more surgical fix for the reported bug, but it touches every caller of markdownWithExtractedHeading and would leave the embed title behaving differently from the standalone notebook title for the prose-before-heading case. This PR takes option A to keep the two title code paths consistent; option B remains available if we later prefer to track the documented wording exactly.

Tests

Two smoke-all tests under tests/docs/smoke-all/2026/06/30/:

• 14577.qmd — the reported case: an embed of a notebook whose only cell is a #-comment code cell, asserting the Source link uses the filename and never the comment.
• embed-heading-after-content.qmd — the general alignment: an embed of a notebook whose only heading has content before it, asserting the Source link falls back to the filename rather than the heading.

Two incidental test-infra cleanups came out of writing that test:

• render-embed.test.ts teardowns now use safeRemoveSync instead of raw Deno.removeSync, so a teardown no longer throws (and aborts, leaving stale state) when a preview artifact is already gone.
• smoke-all's postRenderCleanup now removes recursively, so a registered cleanup entry can be a *_files support directory and not only a single file.

Fixes #14577

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[cderv]

I am putting that as a draft, because maybe option B is better...

I need to think about it.