fix(pptx): convert soft line breaks instead of emitting a vertical tab

[Lukas (L4XB)]

Summary

Shift+Enter inside a PowerPoint paragraph is an <a:br/> — one paragraph, two
lines — and python-pptx reports it as \v. PptxConverter passes shape.text,
cell.text, the notes text and the chart title straight through, so that
vertical tab lands in the Markdown:

>>> MarkItDown().convert("address.pptx").markdown
'<!-- Slide number: 1 -->\nADDRESS LINE ONE\x0bADDRESS LINE TWO'

A vertical tab is a C0 control, not Markdown. A renderer drops it, so the two
lines run together (ADDRESS LINE ONEADDRESS LINE TWO), and nothing downstream
treats it as whitespace either — a splitter sees one long line and a tokenizer
sees one strange token. Address blocks, signature blocks and "name / title"
boxes are where this shows up, which is most of the text in a deck's smaller
shapes.

After:

<!-- Slide number: 1 -->
# TITLE ONE TITLE TWO
ADDRESS LINE ONE
ADDRESS LINE TWO
| HEADER A HEADER B | H2 |
| --- | --- |
| CELL LINE ONE CELL LINE TWO | C2 |

### Notes:
NOTE LINE ONE
NOTE LINE TWO

What changed

One helper, applied where the text enters: \v becomes \n. A shape's own
paragraphs already arrive joined with \n, so a soft break now reads the same
way, and the table path needs nothing special — the newline goes into the HTML
the table is built from, where it is ordinary whitespace and collapses to a
space, so the row stays one row.

A slide title is the one place a newline cannot go: an ATX heading ends at the
line break, which would leave the rest of the title as body text. Titles
therefore collapse their line breaks to spaces. That also repairs the
already-broken case of a multi-paragraph title, which produces
# First\nSecond today.

Tests

test_pptx_soft_line_breaks_are_not_vertical_tabs in
packages/markitdown/tests/test_module_misc.py builds a deck with a soft break
in a title, a text box, a header cell, a body cell and the speaker notes, then
asserts no \v survives and each one reads correctly.

$ pytest packages/markitdown/tests/ -q
893 passed, 14 skipped

$ pytest -k pptx                      # against the current converter
FAILED test_pptx_soft_line_breaks_are_not_vertical_tabs
1 failed, 4 passed

The 893 include the reference test.pptx vector, so the title change is covered
against a real deck as well as the synthetic one.

black==23.7.0 (the pinned pre-commit version) leaves both files unchanged.