[Repo Assist] fix(tests): resolve 24 pyright standard-mode errors in test_scheduler

[dbrattli]

Fix 24 type errors detected when running pyright in standard (non-strict) mode on tests/test_scheduler/:

• test_catchscheduler / test_virtualtimescheduler: add @classmethod to VirtualTimeScheduler.add overrides (base method is a classmethod)
• test_currentthreadscheduler / test_immediatescheduler / test_timeoutscheduler: annotate [None]-initialised lists with the correct element type (list[SchedulerType | None]) to avoid setitem type errors
• test_eventloop/test_asyncioscheduler: widen callback parameter from AsyncIOScheduler to abc.SchedulerBase to match ScheduledAction protocol
• test_eventloop/test_ioloopscheduler, test_twistedscheduler, test_mainloop/test_gtkscheduler: add # type: ignore[import-untyped] to optional-dependency imports (tornado, twisted, gi)
• test_eventloopscheduler: annotate endtime as datetime | None; add assert is not None guard before arithmetic; fix schedule_periodic action to accept only a state argument (matching ScheduledPeriodicAction)
• test_newthreadscheduler: fix action parameter and return types to match ScheduledPeriodicAction[int] (state: int | None) -> int | None
• test_scheduleditem: align ScheduledItemTestScheduler override signatures with Scheduler base class (add default state=None, correct return types)

pyproject.toml: update comment to reflect current status.

[coveralls]

coverage: 93.448% (-0.001%) from 93.449% — repo-assist/eng-pyright-test-scheduler-2026-04-17-762f95f7e6558b77 into master