Turn a skipped vcenter 2D plot check into a deterministic regression test

[cvanelteren]

ultraplot/tests/test_2dplots.py has skipped tests with notes like "not sure what this does" around vcenter / auto-diverging behavior. A good first step is to take one of those cases and make the expected behavior explicit.

This issue is intentionally scoped for newcomers to the project. It is low complexity, focused on a single test case, and should help someone get familiar with the 2D plotting API and the current test conventions.

Suggested scope:

• pick one skipped case such as test_colormap_vcenter() or test_autodiverging2()
• write down the intended behavior in the test docstring or assertions
• replace the skipped placeholder with deterministic assertions or a narrowly scoped image-comparison test

Acceptance criteria:

• at least one skipped vcenter / auto-diverging test is unskipped
• the expected behavior is clearly documented in the test itself
• the new test passes reliably in CI