fix(contentstore): guard downstream_customized copy on paste [FC-0123]

[navinkarkera]

Description

Fix clipboard paste crash in upstream-link setup when pasted block does not expose xml_attributes.

Before change, paste flow always read temp_xblock.xml_attributes["downstream_customized"]. Blocks like DragAndDropBlockWithMixins do not have xml_attributes, so Studio raised AttributeError and paste failed.

Change now:

• Read downstream_customized from parsed block field first (getattr(temp_xblock, "downstream_customized", [])).
• Read xml_attributes only when block exposes it.
• Set downstream_customized only when block has field.

Impact:

• Course Author: paste of blocks like drag-and-drop no longer crashes.

Supporting information

• Private-ref: https://tasks.opencraft.com/browse/FAL-4331
• Discovered while working on: Bug: Drag and drop editor/preview keeps loading frontend-app-authoring#1749

Testing instructions

1. In any library, create drag-and-drop-v2 block
2. Copy block.
3. Paste block into a course unit.
4. Verify paste succeeds (no 500 in CMS logs).
5. Verify pasted block opens and renders in unit.

Deadline

ASAP

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[ChrisChV]

Why has the use of json.loads been removed? If this value is a JSON string, we would be introducing a bug if we didn't use json.loads.

[navinkarkera]

You are right, it could be string as well. I'll update it.

Updated: 62bf50f

[ChrisChV]

Looks good, thanks! 👍

• I tested this: I followed the testing instructions
• I read through the code and considered the security, stability and performance implications of the changes.
• I tested that the UI can be used with a keyboard only (tab order, keyboard controls).
• Includes tests for bugfixes and/or features added.
• Includes documentation
• Includes fixtures that create objects required for manual testing.