ci: install graphviz in unittest/codecov jobs

test_dotfile() exercises the real dot->PDF rendering path via
showgraph(), but only the docs job installed graphviz -- unittest
and codecov didn't, so `dot` was silently missing there. The previous
version of this test had no assertion on the result, so this went
unnoticed; the new assertion (mock_open.assert_called_once(), added
in this same PR) correctly caught it: CI failed with "dot: not
found", showgraph()'s subprocess call returned nonzero, and
webbrowser.open() was consequently never reached.

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

Author: petercorke

.github/workflows/ci.yml

@@ -22,6 +22,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install .[dev]
+          sudo apt-get update
+          sudo apt-get install -y graphviz
       - name: Test with pytest
         run: pytest
 
@@ -59,6 +61,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -e .[dev]
+          sudo apt-get update
+          sudo apt-get install -y graphviz
       - name: Run coverage
         run: |
           pytest --cov=pgraph --cov-report=xml:coverage.xml