🧹 Refactor deeply nested logic in findXmlElementRanges

[MarsLuay]

🧹 What: Refactored deeply nested control flow in findXmlElementRanges in src/docxTableCellFontSizePreserver.ts.
💡 Why: Early guard clauses and flattened conditionals improve code readability, maintainability, and cognitive complexity.
✅ Verification: Verified with npm run typecheck, focused test execution on tests/docx-table-font-size-fixture.test.mjs, and full npm test suite passing.
✨ Result: Improved readability and maintainability without altering existing runtime behavior.

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PR created automatically by Jules for task 3779126507128297657 started by @MarsLuay

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