fix: handle PPTX SVG images without a rasterized fallback

[guoyu-wang]

PowerPoint stores an SVG picture as an <a:blip> whose r:embed points to a raster PNG fallback plus an asvg:svgBlip extension for the SVG. When a picture has no raster fallback, the <a:blip> has no r:embed, so python-pptx's shape.image raises ValueError("no embedded image"). This caused PptxConverter to fail the entire conversion.

Resolve the SVG part directly from the svgBlip extension via a new _get_image_info helper, and guard _is_picture against the exception (hasattr only swallows AttributeError). Add unit tests plus a small synthetic PPTX fixture covering the SVG-without-fallback case.

[guoyu-wang]

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