PPTX conversion takes hours on charts bound to a whole Excel column #2701

Converting a .pptx with a chart can take hours. #2227 removed the extra loop in _convert_chart_to_markdown, but series.values in python-pptx is itself quadratic: for each declared point it runs an XPath search over all of the series' points.

When a series is bound to a whole column, its cache can declare many points, however few cells hold data. A deck of ours had
two such scatter charts with about 20,600 real points each. With markitdown 0.1.8 and python-pptx 1.0.2 each lookup took about 7 ms, so about 2 hours per chart.

python-pptx has had no commits since 2024, so I'd fix it here by reading each series' points once. PR to follow.