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](https://github.com/Infleqtion/client-superstaq/actions/runs/22884111478)
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](https://github.com/Infleqtion/client-superstaq/actions/runs/22887601930/job/66403774452),
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

---------

Signed-off-by: Bharath <bharath.thotakura@infleqtion.com>
Co-authored-by: richrines1 <85512171+richrines1@users.noreply.github.com>