Fix PPTX OCR converter emitting literal backslash-n instead of newlines

[SpongeBob (As9xm)]

_pptx_converter_with_ocr.py used escaped \\n literals in slide headers, text frames, notes, tables, and charts, so output contained literal \n text instead of line breaks.
This change replaces all 10 occurrences with real \n newlines to restore correct markdown formatting.

[SpongeBob (As9xm)]

@microsoft-github-policy-service agree

[Adam Fourney (afourney)]

Thank you for identifying and fixing the literal backslash-n output paths.

#2506, included in v0.1.8, replaced the duplicated PPTX OCR formatting code with the core presentation pipeline. The affected slide, notes, table, and chart formatting now comes from that implementation, superseding these replacements.

Closing this PR with thanks. Please use markitdown==0.1.8 with markitdown-ocr==0.1.1. Please reopen this PR if it still addresses a case that the current implementation misses, with a reproduction or a description of the remaining difference.