fix(pptx): handle unsupported 3-D chart types without crashing

[ceberam]

Resolves #3970

Problem

A .pptx containing a 3-D chart element (e.g. c:bar3DChart) caused the
entire document conversion to fail with:

RuntimeError: Pipeline SimplePipeline failed
  ← AttributeError: 'lxml.etree._Element' object has no attribute 'iter_sers'

python-pptx has no registered custom element class for 3-D chart types, so
chart.series raises AttributeError on the raw lxml element. One affected
slide was enough to lose the whole deck.

Fix

In _handle_chart, the call to _chart_to_table_data is now guarded with the
same try/except pattern already used one line above for chart.chart_type:

try:
    table_data = self._chart_to_table_data(chart)
except (AttributeError, ValueError, KeyError) as exc:
    _log.warning("Could not extract chart data: %s", exc)
    table_data = None

With table_data = None the rest of _handle_chart already handles the
graceful path — the chart is emitted as a PictureItem with its classification
and caption (if any), tabular_chart is None, and all other slide content
is preserved.

Tests

The existing pptx_chart.pptx fixture was extended with a second slide
containing a c:bar3DChart element (patched in-place from a normal
c:barChart, which is what PowerPoint writes for 3-D bar/column charts).

• test_chart_parsed_as_classified_picture_with_data — updated to cover both
  slides: the 2-D chart (slide 1) is verified to carry full tabular data; the
  3-D chart (slide 2) is verified to produce a PictureItem with
  tabular_chart = None and no crash.
• Related chart tests updated to reflect the fixture now having two pictures.
• Groundtruth files for pptx_chart.pptx regenerated.

Checklist:

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

[github-actions]

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[dolfim-ibm]

lgtm