feat: add plugin slot for content iframe error component

[jansenk]

Adds a plugin slot, org.openedx.frontend.learning.content_iframe_error.v1 to enable customization via plugins of the error page shown when the unit iframe fails to load.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.39%. Comparing base (6892633) to head (215604d).
⚠️ Report is 27 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1771   +/-   ##
=======================================
  Coverage   90.38%   90.39%           
=======================================
  Files         344      345    +1     
  Lines        5811     5817    +6     
  Branches     1355     1398   +43     
=======================================
+ Hits         5252     5258    +6     
+ Misses        542      540    -2     
- Partials       17       19    +2     

☔ 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.

[Copilot]

Pull Request Overview

This PR adds a new plugin slot to enable customization of the error page displayed when content iframe fails to load in the learning application. The plugin slot allows developers to replace or modify the default error page with custom implementations.

• Creates a new ContentIFrameErrorSlot plugin slot component with slot ID org.openedx.frontend.learning.content_iframe_error.v1
• Replaces the direct usage of ErrorPage component with the new plugin slot in the ContentIFrame component
• Provides documentation and configuration examples for the new plugin slot

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/plugin-slots/ContentIFrameErrorSlot/index.tsx	Creates the new plugin slot component that wraps ErrorPage as default content
src/plugin-slots/ContentIFrameErrorSlot/README.md	Provides documentation and usage examples for the plugin slot
src/courseware/course/sequence/Unit/ContentIFrame.jsx	Updates error handling to use the new plugin slot instead of ErrorPage directly