[#1397] allow graphviz tests without dot installed

Update the Graphviz message-flow tests so CI environments without the
Graphviz dot executable still exercise a positive code path instead of
skipping. The renderer test now validates DOT output when dot is absent,
and the scenario helper tests explicitly cover both the available and
unavailable Graphviz paths.

This avoids pytest-error-for-skips turning optional Graphviz coverage into
CI failures on platforms where dot is not installed.

Author: schaubh

src/utilities/tests/test_SimulationBaseClass.py

@@ -204,10 +204,7 @@ def test_GetMessageConnectionDot_contains_graphviz_edges():
 
 
 def test_ShowMessageConnectionFigure_graphviz_writes_file(tmp_path):
-    """Check that the Graphviz renderer writes a rendered file when available."""
-    if shutil.which("dot") is None:
-        pytest.skip("Graphviz dot executable is not available.")
-
+    """Check that the Graphviz renderer writes an output file."""
     scSim = SimulationBaseClass.SimBaseClass()
     testProcess = scSim.CreateNewProcess("testProcess")
     simulationTimeStep = macros.sec2nano(1.0)  # [ns]
@@ -222,23 +219,28 @@ def test_ShowMessageConnectionFigure_graphviz_writes_file(tmp_path):
     scSim.AddModelToTask("testTask", cModule)
     scSim.AddModelToTask("testTask", cppModule)
 
-    outputPath = tmp_path / "messageConnections.svg"
+    graphvizIsAvailable = shutil.which("dot") is not None
+    outputPath = tmp_path / (
+        "messageConnections.svg" if graphvizIsAvailable else "messageConnections.dot"
+    )
     renderedPath = scSim.ShowMessageConnectionFigure(
         renderer="graphviz",
         fileName=str(outputPath),
+        graphvizFormat="svg" if graphvizIsAvailable else "dot",
         includeUnlinked=False,
     )
 
     assert renderedPath == str(outputPath)
     assert outputPath.exists()
-    assert outputPath.read_text().lstrip().startswith("<?xml")
+    outputText = outputPath.read_text()
+    if graphvizIsAvailable:
+        assert outputText.lstrip().startswith("<?xml")
+    else:
+        assert "digraph BSKMessageConnections" in outputText
 
 
 def test_saveScenarioGraphvizFigure_uses_documentation_image_path(tmp_path):
     """Check that Graphviz scenario figures are saved to the docs image path."""
-    if shutil.which("dot") is None:
-        pytest.skip("Graphviz dot executable is not available.")
-
     docsPath = tmp_path / "docs" / "source"
     docsPath.mkdir(parents=True)
     scenarioPath = tmp_path / "examples"
@@ -247,12 +249,13 @@ def test_saveScenarioGraphvizFigure_uses_documentation_image_path(tmp_path):
     scSim = MagicMock()
     scSim.ShowMessageConnectionFigure.return_value = str(expectedPath)
 
-    renderedPath = unitTestSupport.saveScenarioGraphvizFigure(
-        "messageFlow",
-        scSim,
-        str(scenarioPath),
-        includeUnlinked=False,
-    )
+    with patch("Basilisk.utilities.unitTestSupport.shutil.which", return_value="dot"):
+        renderedPath = unitTestSupport.saveScenarioGraphvizFigure(
+            "messageFlow",
+            scSim,
+            str(scenarioPath),
+            includeUnlinked=False,
+        )
 
     assert renderedPath == str(expectedPath)
     assert expectedPath.parent.exists()
@@ -265,7 +268,7 @@ def test_saveScenarioGraphvizFigure_uses_documentation_image_path(tmp_path):
     )
 
 
-def test_saveScenarioGraphvizFigure_skips_without_graphviz(tmp_path):
+def test_saveScenarioGraphvizFigure_returns_none_without_graphviz(tmp_path):
     """Check that optional Graphviz scenario figures do not require Graphviz."""
     docsPath = tmp_path / "docs" / "source"
     docsPath.mkdir(parents=True)