test(visualization): isolate optional dependency mocks (#897)

* 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.

---------

tests/visualization/_plotly_doubles.py

@@ -0,0 +1,29 @@
+"""Shared helper for visualization tests.
+
+The visualization modules treat Plotly as optional: they bind ``px``, ``go`` and
+``make_subplots`` to ``None`` when the import fails, and raise ``ProcessingError``
+from ``_check_dependencies()``. Tests that exercise a Plotly-backed path need
+those names to be usable, otherwise ``patch("...go.Figure")`` fails on ``None``
+and the visualizers refuse to run.
+
+``plotly_doubles`` fills in a double for each alias that is ``None``, so the
+tests describe their own requirements instead of depending on whether Plotly
+happens to be installed. When Plotly is installed the aliases are left alone and
+the patches keep asserting against the real attribute names.
+"""
+
+from contextlib import ExitStack, contextmanager
+from unittest.mock import MagicMock, patch
+
+PLOTLY_ALIASES = ("px", "go", "make_subplots")
+
+
+@contextmanager
+def plotly_doubles(*modules):
+    """Stand in for the module-level Plotly aliases that are unavailable."""
+    with ExitStack() as stack:
+        for module in modules:
+            for alias in PLOTLY_ALIASES:
+                if getattr(module, alias, "unused") is None:
+                    stack.enter_context(patch.object(module, alias, MagicMock()))
+        yield