fix(pptx): process slide shapes in visual reading order

[Adityav20]

Description:

This PR improves PPTX parsing by processing slide shapes in visual reading order instead of PowerPoint’s internal creation/z-order.

Previously, when a slide had multiple subheadings with separate bullet text boxes, bullet points could be grouped under the wrong subheading if the PPTX shape order did not match the visible slide layout.

The PPTX backend now sorts shapes top-to-bottom and left-to-right, with a small row tolerance for near-aligned objects. This keeps visually related subheadings and bullet lists together during extraction.

A regression test was added for a slide where the second bullet textbox is stored before its subheading internally, but appears below it visually.

Issue resolved by this Pull Request:
Resolves #1324

Checklist:

• [ x ] Documentation has been updated, if necessary.
• [ x ] Examples have been added, if necessary.
• [ x ] Tests have been added, if necessary.

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Codecov Report

❌ Patch coverage is 5.71429% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/backend/mspowerpoint_backend.py	5.71%	33 Missing ⚠️

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