Add python3-venv to base image (Mason needs ensurepip for venv creation)

python3-pip alone wasn't enough — Debian trixie ships python3 and
python3-pip as separate packages from python3.13-venv. Mason creates
a venv per package then pip-installs into it. Without python3-venv,
'python3 -m venv' fails with 'ensurepip is not available' and every
Mason Python package (ruff, ansible-lint, etc.) errors on every nvim
start.

Adding python3-venv (which pulls in ensurepip + pip-whl + setuptools-whl)
completes the chain: venv creation works, pip is available inside the
venv, Mason installs succeed.

Dockerfile

@@ -43,6 +43,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     g++ \
     rsync \
     python3-pip \
+    python3-venv \
     && ln -s /usr/bin/fdfind /usr/local/bin/fd \
     && rm -rf /var/lib/apt/lists/*