fix(pptx): keep converting when a shape type is unrecognized

[Aimery Barrault (aImErYbArRaUlT)]

Fixes #2569.

PptxConverter reads shape.shape_type while walking a slide (in _is_picture, _is_table, and the group-shape check). If python-pptx can't map the shape, that attribute access raises NotImplementedError: Shape instance of unrecognized shape type, which propagates out of convert() and fails the entire presentation - every slide lost because of one shape.

What was wrong

One unrecognized shape takes the whole deck down. On docling's fixture the error is:

PptxConverter threw NotImplementedError with message: Shape instance of unrecognized shape type

and nothing is returned, even though the surrounding slides are ordinary.

The fix

A small _shape_type_or_none() helper that returns the shape's MSO type, or None when shape.shape_type raises NotImplementedError. The three type checks call it instead of reading shape.shape_type directly, so an unrecognized shape is simply "none of the handled types" and falls through. It doesn't crash, and because the fall-through reaches the has_text_frame branch, any text the shape carries is still recovered rather than dropped. No change for well-formed decks: for a recognized shape the helper returns the identical value.

Evidence

Verified against docling's fixture for exactly this case, powerpoint_unrecognized_shape.pptx:

• Before: PptxConverter threw NotImplementedError ... Shape instance of unrecognized shape type; nothing returned.
• After: all 3 slides convert, and the unrecognized shape's own text ("Text in an unrecognized shape") is recovered.
• Well-formed decks are unaffected: for a recognized shape the helper returns the identical value, so their output is byte-identical.

Tests

Added tests/test_pptx_unrecognized_shape.py, built on a real, self-authored fixture: a <p:sp> with no placeholder, no preset or custom geometry, and a <p:cNvSpPr> without txBox="1" - exactly the element python-pptx cannot map - injected into a slide's shape tree. It asserts the fixture genuinely raises NotImplementedError (so it stays a real test of this path), that _shape_type_or_none returns None, that the shape is treated as neither a picture nor a table, and end-to-end that one such shape does not abort the deck: the presentation still converts and the surrounding text survives.

Scope

Robustness only. Targets the NotImplementedError from an unrecognized shape_type; no change to output for well-formed decks. Independent of the separate reading-order change (filed next).

Checklist

• Tests have been added
• Documentation has been updated, if necessary
• Examples have been added, if necessary

[Aimery Barrault (aImErYbArRaUlT)]

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