Omit MOVETO lines from nearest contour logic

[ksunden]

Closes #27333

PR summary

I would like to simplify down the reproducer further and add a test, but I think the code change itself is largely good.

Just uses path._iter_connected_components() to break apart the connected subpaths so that the MOVETO lines don't get considered as close.
There is a small amount of bookkeeping to make sure that the reported index includes all of the subpaths already processed.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[ksunden]

Test is now written, I decided not to make it an image test, as all I really need to check is that the correct contour is chosen, and that can be done with a simple assertion.

However here are the images produced by that code if reviewers are interested:

Before the change:

With this patch:

The main point being that the MOVETO line of the 100 contour had been selected, but now it (correctly) selects the 0 contour.