fix(convert): strip auto <p:style> theme refs from PDF→PPTX shapes

[nelsonduarte]

Problem

User reported that slide 1 of his UFCD 1492 deck rendered correctly in PowerPoint (dark dotted background, blue UFCD 1492 badge, title, etc.) but slide 6 looked blank — only the floating text was visible. The PPTX inspect script we wrote confirmed the slide's XML contained 13 valid <p:sp> elements with correct <a:solidFill><a:srgbClr val="..."/></a:solidFill> for each rectangle. So the file was fine; PowerPoint was deciding not to render those specific rectangles.

The user's slide 6 XML showed each shape carrying both:

<p:spPr>
  <a:solidFill><a:srgbClr val="0E172A"/></a:solidFill>   <!-- explicit dark fill -->
  <a:ln><a:noFill/></a:ln>
</p:spPr>
<p:style>
  <a:lnRef idx="1"><a:schemeClr val="accent1"/></a:lnRef>
  <a:fillRef idx="3"><a:schemeClr val="accent1"/></a:fillRef>
  <a:effectRef idx="2"><a:schemeClr val="accent1"/></a:effectRef>
  <a:fontRef idx="minor"><a:schemeClr val="lt1"/></a:fontRef>
</p:style>

Root cause

python-pptx's slide.shapes.add_shape(...) always emits an auto <p:style> block with theme presets (effectRef "moderate effect 2", fillRef "intense fill 3"). Per the OOXML spec, <p:spPr> should win over <p:style>, but PowerPoint's renderer applies the theme effect on top in some shape-mix configurations (apparently triggered on slide 6 by the combination of overlapping rectangles + 21 textboxes), masking the explicit colour entirely. LibreOffice and python-pptx-based inspection both correctly use the explicit fill, which is why our diagnostics looked clean while PowerPoint silently rendered blank.

Fix

Strip the auto-generated <p:style> element after add_shape() returns. Every drawing-extraction shape now emits only <p:spPr> so PowerPoint has to use the explicit colour we set:

style_el = shape._element.find(qn("p:style"))
if style_el is not None:
    shape._element.remove(style_el)

Test plan

• Smoke test on Linux Py3.14: a 1-page PDF with 3 filled rectangles produces a PPTX with 4 shapes total (incl. white full-slide bg) and 0 <p:style> blocks. Before this commit every shape carried the theme reference.
• User pulls + re-converts UFCD 1492 PDF and opens slide 6 in PowerPoint: dark navy header / blue separator / dark code block / 4 colored card backgrounds should all be visible now.
• Slide 1 (which already rendered correctly) should still render correctly — same fill colour, just without the theme override.

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