feat: Add V2 of header actions slot

[ChrisChV]

The showUserDropdown prop gated both the LearningHeaderActionsSlot and the AuthenticatedUserDropdown, making it impossible to hide the user dropdown without also hiding all the action items, and impossible for third-party plugins attached to that slot to render independently of the user dropdown's visibility.

This PR introduces a new org.openedx.frontend.layout.learning_header_actions.v2 slot:

• v2 always renders, regardless of showUserDropdown. Use this slot ID for new plugin configurations that should render even when showUserDropdown is false.
• v2 wraps the existing v1 slot (notifications tray + help link), which keeps its original behavior unchanged — only rendered when showUserDropdown is true — preserving backwards compatibility for existing consumers of v1.

This PR does not change any default, out-of-the-box visual behavior. With no custom plugin configuration,
the header looks and behaves exactly as it does on main, including with showUserDropdown={false}.
The only thing this PR unlocks is the ability for plugin authors to opt into .v2 so their own widgets render
independently of showUserDropdown.

A follow-up DEPR ticket will propose removing v1 once its default content (notifications tray + help link) can move into v2 unconditionally.

Supporting information

• Related issue: Learning Header's user menu slots can't fully hide/replace it. #675
• Related PR and discussion: fix: showUserDropdown hiding LearningHeaderActionsSlot #676
• DEPR ticket: [DEPR]: Learning Header Actions Slot V1 #681

Testing instructions

Important: Before starting, ensure you have a Tutor environment with the Learning MFE in development mode running.

1. Clone the repo. From the main branch, run npm ci && npm run build.
2. From the Learning MFE, run npm install "../path/to/this/repo" --legacy-peer-deps.
3. Add the following to frontend-app-learning/env.config.jsx to set showUserDropdown: false:

   import { PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';

   const hideUserMenu = (widget) => {
    widget.content = {
      ...widget.content,
      showUserDropdown: false,
    };
    return widget;
  };

   export default {
     pluginSlots: {
       'org.openedx.frontend.layout.header_learning.v1': {
         keepDefault: true,
         plugins: [
           {
             op: PLUGIN_OPERATIONS.Modify,
             widgetId: 'default_contents',
             fn: hideUserMenu,
           },
         ],
       },
     },
   };

4. Add the following to frontend-app-learning/module.config.js:

module.exports = {
  localModules: [
    {
      moduleName: '@edx/frontend-component-header',
      dir: '../frontend-component-header', // adjust this relative to your repo
    },
  ],
};

5. Run the Learning MFE in dev mode with main checked out on this repo. Confirm that with showUserDropdown: false, the user dropdown, notifications, and "Help" link are hidden (pre-PR behavior).
6. Switch this repo to this PR's branch and run npm run build
7. Confirm the header still looks the same as step 5 — no regression.

[openedx-webhooks]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.33%. Comparing base (e380431) to head (d113c03).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #682      +/-   ##
==========================================
+ Coverage   73.07%   73.33%   +0.25%     
==========================================
  Files          60       61       +1     
  Lines         520      525       +5     
  Branches      110      110              
==========================================
+ Hits          380      385       +5     
  Misses        137      137              
  Partials        3        3              

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