Install demo extra explicitly instead of relying on group overlap

The no-PETSc CI leg installed gmsh/pyvista by hand to top up what the
test group happened to already cover from the demo extra
(matplotlib/networkx/scipy/numba). That overlap is incidental and
would silently break if the test group's contents ever changed;
install 'python/[demo]' directly so the intent is explicit.

Also guard the unguarded slepc4py import in
demo_half-loaded-waveguide.py, matching the try/except pattern already
used for pyamg and pyvistaqt, rather than adding slepc4py to the demo
extra (which would force a PETSc build on every demo-extra install for
one demo that needs it).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: jhale

.github/workflows/ccpp.yml

@@ -150,19 +150,18 @@ jobs:
 
       - name: Build Python interface
         run: >
-          pip install --group python/pyproject.toml:test 'python/'
+          pip install --group python/pyproject.toml:test 'python/[demo]'
           --check-build-dependencies
           --no-build-isolation
           --config-settings=cmake.build-type="Developer"
 
       - name: Check Python configuration
         run: python -c "from mpi4py import MPI; import dolfinx; assert not dolfinx.has_petsc; assert not dolfinx.has_petsc4py; assert dolfinx.has_superlu_dist"
 
-      - name: Install gmsh and pyvista (and dependencies)
+      - name: Install system dependencies for gmsh and pyvista
         run: |
           sudo apt-get install libglu1-mesa libgl1 libxrender1 libxcursor1 libxft2 libxinerama1
           sudo apt-get install libegl1
-          pip install gmsh pyvista
 
 
       - name: Run demos (Python, serial)

python/demo/demo_half-loaded-waveguide.py

@@ -48,7 +48,12 @@
 from petsc4py import PETSc
 
 import numpy as np
-from slepc4py import SLEPc
+
+try:
+    from slepc4py import SLEPc
+except ModuleNotFoundError:
+    print("This demo requires slepc4py.")
+    sys.exit(0)
 
 import ufl
 from basix.ufl import element, mixed_element