Fix Quarto Jupyter engine: install PyYAML

Quarto's own jupyter wrapper (/opt/quarto/share/jupyter/jupyter.py ->
notebook.py) does 'from yaml import safe_load', so executing a {python} cell
failed with ModuleNotFoundError: No module named 'yaml'. The minimal jupyter
stack didn't pull PyYAML in (psutil/ipython already come via ipykernel), so
add pyyaml explicitly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server-ce/Dockerfile-base

@@ -74,7 +74,8 @@ RUN mkdir -p /opt/quarto-extensions \
 # Quarto runs ```{python}``` cells through a Jupyter kernel. It uses the system
 # python3 it detected (/usr/bin/python3), so Jupyter must be installed there.
 # We install only the headless execution stack Quarto needs (jupyter-client +
-# nbclient/nbformat + the ipykernel kernel), not the notebook/lab servers, and
+# nbclient/nbformat + the ipykernel kernel + pyyaml, which Quarto's own
+# /opt/quarto/share/jupyter wrapper imports), not the notebook/lab servers, and
 # register a system-wide "python3" kernelspec under /usr/local/share/jupyter so
 # it is discoverable regardless of HOME/XDG. Noble's Python is externally
 # managed (PEP 668), hence --break-system-packages in this controlled image.
@@ -83,7 +84,7 @@ RUN mkdir -p /opt/quarto-extensions \
 RUN apt-get update \
 &&  apt-get install -y python3-pip \
 &&  pip3 install --no-cache-dir --break-system-packages \
-      jupyter-core jupyter-client nbclient nbformat ipykernel \
+      jupyter-core jupyter-client nbclient nbformat ipykernel pyyaml \
 &&  python3 -m ipykernel install --prefix /usr/local --name python3 --display-name "Python 3" \
 # Bundle the common scientific-Python stack so most decks "just work" without
 # any per-project install. matplotlib renders headless (Agg) automatically.