markitdown-ocr: PPTX converter emits literal \n sequences in Markdown output

[MontesanoDev]

PptxConverterWithOCR uses "\\n" instead of "\n" in multiple output paths.
As a result, converted PPTX files contain literal backslash-n sequences rather
than Markdown line breaks.

The OCR PPTX tests currently encode this malformed output as expected behavior.

Additionally, the optional LLM caption path imports ._llm_caption, but the
module exists in markitdown.converters, not in markitdown_ocr.