fix: DocLang deserialization of XML-sensitive OTSL cell content

[PeterStaar-IBM]

Summary

Fix DocLang round trips for OTSL table cells containing XML-sensitive text such as &, <, >, quotes, and entity-like
strings.

Changes

• Escape DOM text nodes when reconstructing XML fragments for reparsing.
• Preserve semantics for ordinary text and CDATA.
• Safely serialize literal ]]> by splitting it across adjacent CDATA sections.
• Add regression coverage for:
  • EscapeMode.AUTO and EscapeMode.ALWAYS
  • Virtual and explicit cells
  • Nested formatting
  • Multiline, Unicode, and mathematical content
  • Literal entity-like strings such as &

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