sol.graphs() unavailable for collocation problems when min/max bounds are involved

[Kev1CO]

Modify file plot.py

TODO: introduce repeat for the COLLOCATIONS min/max_bounds only for states graphs.
For now the plots in COLLOCATIONS with LINEAR are not giving the right values
nlp.plot[variable].bounds.check_and_adjust_dimensions(n_elements=len(mapping_to_first_index), n_shooting=ns)

[mickaelbegon]

I opened draft PR #1065 to fix the rendering of InterpolationType.LINEAR bounds: they now use linear lines while constant bounds keep their step rendering. A focused regression test checks the interpolated min/max values. The full non-visual plot test module passes locally (6 passed), and git diff --check is clean. @pariterre, the implementation is working locally and is ready for review; CI is now running on the PR.