Reduce the Redux store to the external specialExams reducer

Part of #1946 — Redux → React Query migration (Stage 1). Partial teardown.

Goal: shrink the Redux store to the last external dependency — specialExams.

Tasks

• Reduce store.ts to only specialExams (external, from @edx/frontend-lib-special-exams). All local reducers leave first, including the plugins slice (#2017 — its de-Redux is Stage-1, not a Stage-2 concern).
• This specialExams-only endpoint needs no external dependency — it depends only on learning's own remaining conversions (tabs, courseware, model-store #1977, plugins #2017).
• Final dep removal is separate and library-gated: dropping react-redux / redux / @reduxjs/toolkit, deleting store.ts, and removing AppProvider store= requires the special-exams components to no longer need a host-supplied react-redux Provider + store. The approach for that (self-contained library store vs. React Query conversion) is being investigated in #2018; a concrete frontend-lib-special-exams change is filed only if/when that investigation says so.

Verify: store.ts registers only specialExams; git grep react-redux in learning's own source returns only special-exams-adjacent files; exams still function (timer, gating, access token).