performance: Refactor BasePlot & subclasses to copy less

[dorzhey]

• Closes #3718
• [Tests][plotting] this is a pure refactor of plotting internals, no new behavior, and all existing plotting tests pass locally (the one failing heatmap test also fails on main in my environment).

• Release notes not necessary because: there are no user-facing API changes, only internal performance and memory optimizations.

[dorzhey]

Ah, I failed some of the style checks. I apologize, pre-commit does not work on my environment for some reason, gives error with UTF encoding and tests fail on main

[flying-sheep]

no worries, I fixed it for you

[flying-sheep]

OK, I got rid of the “converting index to string” warnings that obscured the actual test failures, so you should be able to see which tests failed now: https://github.com/scverse/scanpy/actions/runs/16826467121/job/47664011892?pr=3757#step:6:7989