Fix Container Blocks in the New Unit Editor

[salman2013]

Description

Note: The changes in this PR is generated by Claude Sonet 4.6, reviewed by Salman. Based on the implementation plan suggested on the original ticket (openedx/frontend-app-authoring#2620)

The bug details can be found on the ticket: openedx/frontend-app-authoring#2620

As per the proposed implementation plan
In this PR we fixes issues caused by legacy add-component buttons being rendered for non-vertical container types inside the MFE iframe.
We introduce changes to ensure that container blocks rendered inside the MFE only expose component templates that are explicitly allowed by the container XBlock configuration.

frontend-app-authoring changes PR is available: openedx/frontend-app-authoring#3058

Acceptance Criteria

Using the Steps to reproduce mention here (conditional + at least one of xblock-controlled-navigation, xblock-content-restrictions, xblock-extemporaneous-grading):

• All Add New Component buttons function — no silent no-ops on Text / Library Content / Open Response / Advanced.
• Video editor closes cleanly; no "connection error" panel afterwards.
• No oversized grey iframe around Problem/Video editors.
• A container block that declares a restricted allowed_nested_blocks shows only the declared subset, with disabled / single_instance honored if surfaced.
• A container block that does not use StudioContainerWithNestedXBlocksMixin shows the full standard button set and behaves identically to a normal unit.
• No additional API round-trips on container open vs. today.

Testing:

Sandbox for testing: https://github.com/openedx/frontend-app-authoring/pull/3058/checks?check_run_id=75731943406

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Thanks for the pull request, @salman2013!

This repository is currently maintained by @openedx/wg-maintenance-openedx-platform.

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