fix: keep FileNotFoundError catching a missing Graphviz dot

[fgmacedo]

Why

Before 3.2.2, a missing dot raised pydot 4.0.1's FileNotFoundError('[Errno 2] "dot" not found in path.') (checked with PATH=/nonexistent on pydot 4.0.1). #672 replaced it with GraphvizError(StateMachineError), so except FileNotFoundError around diagram generation would stop catching a missing Graphviz in a patch release.

What changes

• A missing Graphviz program raises GraphvizNotFoundError, a subclass of both GraphvizError and FileNotFoundError, with errno == ENOENT. str() is the same message fix: report Graphviz stderr when dot fails to render a diagram #672 introduced.
• GraphvizNotFoundError is exported from statemachine.contrib.diagram next to GraphvizError.
• A dot that exists but cannot run, or that exits non-zero, still raises a plain GraphvizError, which is not an OSError.
• docs/diagram.md documents both ways to catch it (with a doctest), and the Dot and svg diagrams fail to generate without useful error message #649 entry in docs/releases/3.2.2.md mentions the subclass.

Evidence

$ PATH=/nonexistent .venv/bin/python -c 'import pydot; g=pydot.Dot("g"); g.add_node(pydot.Node("a")); g.create_svg()'
FileNotFoundError: [Errno 2] "dot" not found in path.

$ timeout 120 uv run pytest tests/test_contrib_diagram_graphviz.py tests/test_contrib_diagram.py docs/diagram.md --cov=statemachine.contrib.diagram.graphviz --cov-branch --cov-report=term-missing
statemachine/contrib/diagram/graphviz.py      26      0      6      0   100%
173 passed

$ timeout 180 uv run pytest -n 4
2634 passed, 25 skipped, 44 xfailed

New tests run with dot removed from PATH (monkeypatch.setenv("PATH", tmp_path)) through run_graphviz and format(sm, "svg"). They assert pytest.raises(FileNotFoundError), isinstance of GraphvizNotFoundError and GraphvizError, errno, and the exact message.

Not verified

• Windows: not run. CPython raises FileNotFoundError for a missing executable there as well, so it should take the same branch.
• strerror and args differ from pydot's old exception (strerror is None and args == (message,), where pydot gave (2, message)). Only the except FileNotFoundError and errno contracts are kept.

Refs #672
Refs #649

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (73bf57c) to head (e62fc80).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #680   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           53        53           
  Lines         5787      5793    +6     
  Branches       918       918           
=========================================
+ Hits          5787      5793    +6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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