Rendered Markdown diff on YAML frontmatter: edited field shows no diff, unchanged heading marked as changed

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.134.0 (110a328ea54b42367b803ec53ee0bf52ef26b419, arm64)
- OS Version: macOS 26.5 (Darwin 25.5.0), arm64

Steps to Reproduce:

1. Commit a Markdown file that starts with a YAML frontmatter block (`--- name: ... description: ... ---`).
2. Edit the frontmatter's description field.
3. Open the file from the Source Control view.
4. Run **Reopen Editor With…** and choose **Markdown Preview**.

## Actual

The frontmatter renders as a table. The edited cell shows no diff indication at all — the old and new sides look identical, no highlighting, no color — even though the text is different.

Right below, the first heading — same text on both sides — gets a full colored background as if it had changed: dark red on the old side, dark green on the new side.

So the one thing that did change is shown as unchanged, and the one thing that didn't change is shown as changed.

## Expected

The frontmatter table cell highlights the actual change, the same way changes are highlighted outside a table. Unchanged headings keep no diff background.

## Related

#332119 looks related — same area (rendered Markdown diff), also broken on tables.