Issue Description:

Graphviz-java uses Graphviz command line binary (e.g. /usr/bin/dot) if available.

If not available, it falls back to GraphvizV8Engine or GraphvizJdkEngine. However, these fallback engines could be dependent on JDK implementation. (got a report that it doesn't work in Mac OSX)

So installing Graphviz would be a quick fix locally.

But as it might fail with "None of the provided engines could be initialized." depending on environments (so far, not happening in CI though), I'll fix unit tests not to break the test run. Also need some docs for users.

Acceptance Criteria:

• Add a unit test to simulate "None of the provided engines could be initialized." case
• Pass unit tests without Graphviz binary
• Add a note to install Graphviz binary for users in docs