* test(visualization): isolate optional dependency mocks
* test(visualization): stop requiring Plotly in unit tests
Removing the global sys.modules stubs left the tests that patch
`...go.Bar`, or call a visualizer, with nothing standing in for the
module level `px` and `go` aliases. Those are None when Plotly is
missing, so patch resolution and _check_dependencies() both failed.
Add a helper that substitutes a double only for the aliases that are
None, leaving the real module in place when Plotly is installed.
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