Convert the plugin-store override registry off Redux to React context

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

Goal: convert the plugin-store override registry off Redux to a React context / module-level registry, so the plugins reducer can leave src/store.ts.

Context — this is not @openedx/frontend-plugin-framework (FPF). FPF is the declarative PluginSlot system (PluginSlot / DIRECT_PLUGIN / PLUGIN_OPERATIONS), used in src/plugin-slots/* and config-driven — it never touches state.plugins. generic/plugin-store is a separate, bespoke Redux "override-method" registry: registerOverrideMethod stores function references by plugin+method (hence the serializable-check exemption in store.ts), and usePluginsCallback(name, default) folds registered overrides over a default. Its only in-repo consumer is the iframe-URL override in src/courseware/course/sequence/Unit/index.jsx.

Why Stage 1: this is learning's own Redux. Getting redux / react-redux / @reduxjs/toolkit out of learning's package.json must happen before the frontend-base move, so this slice has to go regardless of the special-exams blocker. Only the later reshaping of the override API onto the frontend-base Slot system is Stage 2.

Tasks

• Replace the plugin slice + registerOverrideMethod + usePluginsCallback with a React context / module-level registry.
• Remove the plugins reducer from store.ts (and its ignoredActions serializable exemption).
• Keep the single in-repo consumer (Unit/index.jsx iframe-URL override) working.

Open question (settle when scoped): whether the external registerOverrideMethod dispatch contract can be preserved in Stage 1, or whether that reshaping must wait for the Stage-2 Slot migration — the redux removal itself does not.

Verify: iframe-URL override still applies; plugins reducer gone from the store; no react-redux in the plugin-store.

Prerequisite for #1978's "specialExams-only" store endpoint.