Fix PPTX shape sorting treating top-zero as missing

[SpongeBob (As9xm)]

_pptx_converter.py used if not x.top / x.left for sort keys, so shapes at position 0 were treated as missing and sorted as -inf, corrupting slide reading order.
This change checks is None instead to preserve correct order for zero-positioned shapes.

[Copilot]

🟡 Changes recommended

The equivalent OCR converter remains affected, and regression coverage is missing.

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Pull request overview

Fixes PPTX reading order for shapes positioned at coordinate zero.

Changes:

• Uses explicit None checks for group and slide shape sorting.

File summaries

File	Description
_pptx_converter.py	Preserves zero-valued shape coordinates during sorting.

Review details

Suppressed comments (1)

packages/markitdown/src/markitdown/converters/_pptx_converter.py:203

• Please add a regression test that orders a zero-positioned shape against a negative-positioned shape (and covers the grouped-shape path). Comparing zero only with positive coordinates would pass under the old -inf behavior and would not protect this fix; the repository already has PPTX regression coverage in packages/markitdown/tests.

                    float("-inf") if x.top is None else x.top,
                    float("-inf") if x.left is None else x.left,

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Balanced

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