refactor: decouple discussions data-fetching from trigger into widget prefetch lifecycle

[awais-ansari]

Summary

Decouples the discussions widget's data-fetching side effect from DiscussionsTrigger
into the widget's prefetch lifecycle hook, which SidebarContextProvider calls on mount.

Motivation

Previously, DiscussionsTrigger owned its own useEffect to dispatch
getCourseDiscussionTopics. This had two problems:

1. Data-fetch tied to render — if the trigger didn't render (e.g., widget
   marked unavailable), topics were never loaded.
2. Sidebar flickering — discussionsIsAvailable checked unit-level topic data
   (unit?.id && unit?.enabledInContext), causing the DISCUSSIONS widget to toggle
   availability on every unit navigation and triggering the priority cascade
   (COURSE_OUTLINE fallback → switch back).

Changes

File	Change
widgets/discussions/widgetConfig.js	Add prefetch function; change isAvailable to course-level check (baseUrl && hasDiscussionTab)
widgets/discussions/DiscussionsTrigger.jsx	Remove useEffect, useDispatch, getConfig, ensureConfig, useMemo — trigger is now a pure render component
sidebar/SidebarContextProvider.jsx	Add useEffect to call widget.prefetch() on mount; add useDispatch; remove unit from getAvailableWidgets context
sidebar/SidebarContextProvider.test.jsx	Add mocks for react-redux, @edx/frontend-platform, and thunks
widgets/discussions/DiscussionsTrigger.test.jsx	Pre-dispatch getCourseDiscussionTopics in beforeEach; fix async assertions
sidebar/hooks/useSidebarSync.js	Fix JSDoc: Function → Object for ref params
sidebar/hooks/useResponsiveBehavior.js	Fix JSDoc: Function → Object for ref param
sidebar/README.md	Add prefetch to widget structure; update context object (remove unit); add prefetch docs
sidebar/ARCHITECTURE.md	Add prefetch to SidebarContextProvider responsibilities
widgets/discussions/README.md	Update isAvailable signature; add prefetch section; document two-tier availability

Behavioral Change

• isAvailable scope: Unit-level → course-level. The DISCUSSIONS trigger stays
  visible across the entire course. The trigger/sidebar components still check
  topic?.id && topic?.enabledInContext at render time, showing nothing for units
  without a topic.
• No breaking changes to external widget API — prefetch is optional.

Testing

• All existing sidebar + discussions tests pass (12/12 + 2/2)
• No ESLint errors
• Manual verification: trigger bar stable during unit navigation

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.27%. Comparing base (0664dc3) to head (d992898).

Files with missing lines	Patch %	Lines
...urseware/course/sidebar/SidebarContextProvider.jsx	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1895   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files         343      343           
  Lines        5766     5768    +2     
  Branches     1387     1384    -3     
=======================================
+ Hits         5263     5265    +2     
  Misses        484      484           
  Partials       19       19           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.