fix: restore add-component buttons for content experiment groups

[Anas12091101]

Description

Content experiments (split_test) render no Add New Component panels in the authoring MFE, so Course Authors cannot add content to experiment groups at all. Regression from #38636, which narrowed the guard in renderAddXBlockComponents() to canEdit && !isIframeEmbed — correct for units and problem banks, which the MFE renders natively, but the MFE renders no add buttons for split_test (AddComponent only renders its button list for unit verticals). Each group needs its own panel beside its own children, which only the container page can place.

This restores isSplitTestContentPage to the guard, as it was before 3c5cc6fffd. Deliberately narrower than reverting to !model.isVertical(), so problem banks stay on MFE-native rendering.

The rest of the flow was already iframe-aware and dead since the regression: createComponent() posts addNewComponent/hideProcessingNotification to the parent for split_test, add_xblock.js keeps template menus in the iframe when the parent is split_test, and frontend-app-authoring already handles both messages. Nothing changes server-side — the vertical author view always renders group children with can_add=True, so the panels were in the fragment and only JS removed them. No frontend-app-authoring change is required.

Before	After

Supporting information

• Regressed by Fix Container Blocks in the New Unit Editor #38636; guard last correct in 3c5cc6fffd
• Expected behaviour: managing content experiments
• Related: [DEPR]: Legacy Unit Editor #35261 (legacy unit editor removal)

Testing instructions

Requires the new unit page (legacy_studio.unit_editor disabled — the default) and an asset rebuild, since container.js is bundled into the js/factories/container webpack entry:

npm run build-dev     # or `npm run webpack-dev`; Tutor: tutor dev exec cms npm run webpack-dev

Set up an experiment, if the course has none:

1. Settings → Advanced Settings → add "split_test" to Advanced Module List.
2. Settings → Group Configurations → create an Experiment Group Configuration with two groups.
3. In a unit: Add Component → Advanced → Content Experiment.

Verify the fix:

4. From the unit page, click into the Content Experiment to reach /course/<course-id>/container/<split_test-id>/<subsection-id>.
5. Each group shows an Add New Component panel. (Before this change: no panels.)
6. Click Text in Group A, pick a template → the MFE shows its "Adding…" spinner and the component appears inside Group A. Repeat in Group B and confirm it lands in B. Reload; both persist.
7. Click Problem → the template submenu renders inside the iframe, not as an MFE modal.
8. With v2 libraries enabled, Library Content → the picker opens as an MFE modal over the iframe.

Verify nothing else changed:

9. A normal unit → only the MFE's native add row; no duplicate legacy buttons in the iframe.
10. A Problem Bank container page → still MFE-native buttons only.
11. The legacy container page, /container/<split_test-id> → unchanged (never took the regressed branch).

Note on hard-refreshing: the iframe is a separate cross-origin document and dev builds emit an unhashed container.js, so reload the frame itself or use "Disable cache" — a hard reload of the outer page can leave the old bundle in place.

Automated tests: adds two specs to container_spec.js covering both branches. They have not been executed — karma_cms_webpack.conf.js fails to start (ENOENT ... main_webpack.<hash>.js, #35956) and npm run test-karma skips that suite for the same reason. Verification was manual in Chrome, both directions: regressed bundle → 0 panels / 0 buttons; fixed → 2 panels / 16 buttons; adding a component writes it under the clicked group in the modulestore.

Deadline

None

Other information

No dependent PRs, no MFE change, no migrations, no new settings. Limitation: this keeps the legacy per-group add widgets alive for content experiments; removing the legacy unit editor (#35261) will need per-group add UI rendered natively by the MFE.

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[AhtishamShahid]

Tested manually and confirmed the fix works as described.

Environment: Tutor dev (Python 3.12 image, current master baseline), split_test enabled in Advanced Module List, one Experiment Group Configuration (Group A / Group B) added to a course, rebuilt CMS webpack (npm run webpack-dev) to pick up container.js.

• Content Experiment container page now shows a separate Add New Component panel under both Group A and Group B (pre-fix: zero panels — confirmed by reverting to the parent commit and rebuilding).
• Added a Text component via Group A's panel and a Problem via Group B's panel — each template menu rendered inline in the iframe, not as an MFE modal.
• Reloaded the page — both components persisted (written to the modulestore, not just client-side render) and stayed scoped to their own group only.
• Regression check: a normal (non-split_test) unit still shows a single native MFE "Add a new component" row, no duplicate/legacy buttons.