fix: fall back to block type when XBlock plugin is missing

[rpenido]

Description

StagedContentSerializer.get_block_type_display raised PluginMissingError when the XBlock for the staged content's block type is not installed, so reading the clipboard failed. Because the clipboard is embedded in the course index response (initial_user_clipboard), that made GET /api/contentstore/v1/course_index/<course_key> return 500 for every course the user could open, and /api/content-staging/v1/clipboard/ failed as well — with no API to discard the stale clipboard, the author is stuck.

The clipboard keeps a copy of a block, so the plugin can be removed from the platform after the copy was made (e.g. an operator drops a custom XBlock, or the platform is upgraded and a third-party block goes away).

The fix falls back to the raw block type, which is what openedx.core.djangoapps.xblock.api.xblock_type_display_name already does for v2 libraries:

try:
    component_class = XBlock.load_class(block_type)
except PluginMissingError:
    # We want to be able to give *some* value, even if the XBlock is later uninstalled.
    return block_type

Impacted roles: Course Author

Supporting information

• Related to Bug: course index returns 500 when the user's clipboard holds content from an uninstalled XBlock #39151

Testing instructions

1. Install a third-party XBlock
2. Add it to a course
3. Copy the block to the clipboard
4. Uninstall the plugin
5. Restart the LMS/CMS
6. Create a new course and open it

Before this change, the course index API returns 500; and the course will not render.
Checking out this branch and the course will render.

Deadline

None.

Other information

None.

---

Private ref: FAL-4394

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[samuelallan72]

@rpenido 👍 nice catch, works for me :)

• I tested this: followed the test instructions
• I read through the code
• I checked for accessibility issues
• Includes documentation

[rpenido]

Hi @bradenmacdonald! Could you review this when you have some time?

[bradenmacdonald]

@rpenido Yep! I'll review when I can.