fix(Doclang): improve checkbox serialization & deserialization

[vagenas]

Wrap in <text> if needed.

[github-actions]

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• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[codecov]

Codecov Report

❌ Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/experimental/doclang.py	58.82%	7 Missing ⚠️

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