Books: Table of Content picks up code-block comments as chapter titles

Bug description

Using

```{r, echo = FALSE}
# INITIALIZE CODE
hello <- 42
```

# Summary

In summary, this book has no content whatsoever.

in a Quarto Book chapter results in INITIALIZE CODE becoming the chapter title used in the table of content, although the Summary is still the title on the page. See below screenshot.

Session info

Ubuntu 20.04 with:

$ quarto --version
1.3.11

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.

Start with the minimal .qmd example in the issue and render it as a Quarto Book using the reported case. Trace how the book table of contents identifies chapter titles, then verify that the code-block comment is excluded while the Summary heading remains the page and chapter title.