fix: collapsed warning propagation across siblings (#9566)

[goughjo02]

The basics

• I validated my changes

The details

Resolves

Fixes #9566

Proposed Changes

• Add a regression test for collapsed blocks inheriting warnings from following sibling stacks.
• Update collapsed warning propagation to ignore following siblings.

Reason for Changes

Collapsed blocks should only surface warnings from blocks contained within them. Warnings from following siblings are misleading and don’t represent the collapsed block’s contents.

Test Coverage

• Added a Mocha regression test in tests/mocha/block_test.js.
• Manual verification in playground: collapse a block in a stack where the next sibling has a warning; the collapsed block should not show a warning.

Documentation

No documentation updates needed.

Additional Information

screenshot of the fix (compare with screenshots on issue)

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[gonfunko]

Thanks for catching this, and thanks very much for the fix and test!