fix(pptx): emit a blank line between PowerPoint paragraphs - #2759

PptxConverter writes a body text frame with shape.text, which python-pptx joins with a single \n between paragraphs. A single newline is a CommonMark soft break, so separate paragraphs render as one continuous line:

prs = Presentation()
slide = prs.slides.add_slide(prs.slide_layouts[1])
slide.shapes.title.text = "Quarterly review"
frame = slide.placeholders[1].text_frame
frame.text = "Revenue grew 12% year over year."
frame.add_paragraph().text = "Costs stayed flat."
frame.add_paragraph().text = "Outlook is positive."

Actual:

# Quarterly review
Revenue grew 12% year over year.
Costs stayed flat.
Outlook is positive.

which renders as one paragraph — "…year over year. Costs stayed flat. Outlook is positive."

Expected (separate paragraphs, like the DOCX and HTML converters produce):

# Quarterly review
Revenue grew 12% year over year.

Costs stayed flat.

Outlook is positive.

The trailing-double-space alternative (requested in #1331) cannot work here: _markitdown.py rstrips every output line before joining, so the spaces are gone by the time the Markdown is returned. A blank line is preserved (the normalizer only collapses runs of three or more newlines).

Change

Join the paragraphs of a body text frame with a blank line instead of writing shape.text as-is. Title handling is unchanged.

Test

packages/markitdown/tests/test_pptx.py::test_body_paragraphs_are_separate_markdown_paragraphs builds the deck above, asserts the blank-line separation, and asserts that . Costs stayed flat. does not appear. It fails on main and passes with the change; test_pptx.py is 50 passed.

Related: #1331 (hard breaks in PPTX), #1911 and #2513 (hyperlinks/lists and soft line breaks inside a paragraph) are complementary and still join paragraphs with a single newline.