fix(docx): keep the other equations when one cannot be converted

[Mohammad Hijjawi (MohammadHijjawi97)]

Problem

_pre_process_math converts every equation in word/document.xml (and footnotes/endnotes) in a single pass, and pre_process_docx runs it inside a blanket except Exception that writes the original XML back on any error. Mammoth does not render OMML, so a single equation the OMML converter cannot handle silently removes every equation in the document, with no error and exit code 0.

One construct that triggers this on main today is an m:limLow whose base is not lim/max/min, which pandoc writes for \sup_{x}, \inf_{x} or \underset{x}{y} (do_limlow raises NotImplementedError). The existing tests in test_docx.py already note this "not local to the equation" effect for several other constructs.

Before (document with E=mc, inline a+b, one sup equation, and a block paragraph containing y=2 plus a sup equation):

Energy:

Inline end.

Supremum

After:

Energy:

$$E=mc$$

Inline $a+b$ end.

Supremum

$$y=2$$

Change

_get_omath_tag_replacement catches a conversion failure for that one equation and returns None; _replace_equations then leaves only that equation unconverted, so the rest of the document's equations are still rendered as LaTeX. This complements fixes for individual constructs: whatever the converter does not support yet now costs only the equation that uses it.

Tests

• New test_docx_unsupported_equation_keeps_the_other_equations (fails on main, passes with the fix), covering block, inline, and a block paragraph mixing a convertible and an unconvertible equation.
• tests/test_docx.py: 101 passed, 1 skipped; full packages/markitdown/tests: all pass except test_speech_transcription (no ffmpeg locally, unrelated); markitdown-ocr docx tests pass; black 23.7.0 clean.

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