fix(docx): create separate formula items for each standalone equation

[qianchongyang]

When a paragraph contains multiple oMath elements, previously they were concatenated into a single display block. Now each equation is processed separately and creates its own FORMULA item.

Fixes #3121

[github-actions]

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[mergify]

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

[dolfim-ibm]

Duplicate of #3123