fix(docx): properly escape special markdown characters at start of lines

[saitejabandaru-in]

What does this PR do?

Fixes an issue where DOCX conversion with markitdown fails to escape Markdown special characters at the beginning of a line (e.g. #, >, +), causing plain text that happens to start with these characters to be rendered as Markdown headings, blockquotes, or lists.

Fixes #2157

Changes

Added escaping logic in _markdownify.py's process_text to catch line-starting Markdown block characters and escape them. Also added a pytest to verify the escaping works correctly for strings starting with #.

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