test: reach 100% branch coverage for diagram package

Add tests for extract.py (deep history type, internal transitions with
and without actions, bidirectional compound detection, invalid input,
initial state fallback), dot.py (compound labels with actions, internal
action format, targetless transitions, non-bidirectional anchors), and
__main__.py (via runpy).

Mark unreachable branch in extract.py (history states never have
substates) with pragma: no cover, and add `if __name__` to coverage
exclusions in pyproject.toml.

Author: fgmacedo

pyproject.toml

@@ -116,6 +116,7 @@ exclude_lines = [
     "raise AssertionError",
     "raise NotImplementedError",
     "if TYPE_CHECKING",
+    'if __name__ == "__main__"',
 ]
 
 [tool.coverage.html]

statemachine/contrib/diagram/extract.py

@@ -131,7 +131,7 @@ def _extract_all_transitions(states, getter) -> List[DiagramTransition]:
             result.extend(_extract_all_transitions(state.states, getter))
         for history_state in getattr(state, "history", []):
             result.extend(_extract_transitions_from_state(history_state, getter))
-            if history_state.states:
+            if history_state.states:  # pragma: no cover
                 result.extend(_extract_all_transitions(history_state.states, getter))
     return result