Presently, the daily integration test workflow uses only 2 workers from pytest-xdist and generally can take a while to finish, waiting on some slower tests relative to others. Anticipating #1358, which will slow down the workflow even more, this PR adds pytest-split to break up the (current) total of 44 unit tests from the css and qss integration tests combined into 4 concurrent workflows, each using 2 workers. (Note: the tests are split up evenly across the 4 workflows (called 'groups'), but pytest-split can unevenly group time-consuming tests if given pytest timing info)

Additionally, on a more speculative & hopeful note, this PR also applies pytest-split on the three notebook checks in the regular CI to continue to mitigate against #1129 (with the idea being that each concurrent workflow should now only assign 1 notebook per worker (of course, subject to change if new notebooks are added)). In initial testing, this did not seem to offer much in time reduction due to the small number of notebooks being tested to begin with (& the worker overhead perhaps), but this approach might still be worth experimenting with.

Lastly, to confer any potential speed benefits, this PR also updates all relevant CI jobs to use Python 3.14 from the previous Python 3.13