Add python3-pip to base image for Mason LSP installs

Mason (neovim's package manager) creates a Python venv and runs
'pip install' inside it to install Python-based LSP servers like
ruff and ansible-lint. Debian trixie's python3 package ships without
ensurepip, so the venv has no pip and Mason fails with
'spawn: python3 failed with exit code 1'.

Adding python3-pip to the apt install list gives Mason what it needs.
uv is still available as the preferred user-facing Python tool
manager; pip is here specifically for Mason's internal use.

Dockerfile

@@ -42,6 +42,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     gcc \
     g++ \
     rsync \
+    python3-pip \
     && ln -s /usr/bin/fdfind /usr/local/bin/fd \
     && rm -rf /var/lib/apt/lists/*