fix(pptx): conversion aborts when a slide's title placeholder holds a picture

[lsimons]

What happens

Converting an old (2013-era) .pptx deck raises AttributeError and the whole file is dropped:

ERROR   mdd.converters.pptx: error converting <REDACTED>/<REDACTED>.pptx
Traceback (most recent call last):
  File "src/mdd/converters/pptx.py", line 32, in convert
    convert_pptx(src, dest)
  File "src/mdd/convert/pptx.py", line 421, in convert_pptx
    _render_slide(
    ...
  File "src/mdd/convert/pptx.py", line 304, in _render_slide
    str(title_shape.text).strip()
        ^^^^^^^^^^^^^^^^
AttributeError: 'PlaceholderPicture' object has no attribute 'text'

Why

_render_slide assumes slide.shapes.title is a text placeholder:

https://github.com/schubergphilis/mdd/blob/main/src/mdd/convert/pptx.py#L302-L307

title_shape: Any = slide.shapes.title
slide_title: str = (
    str(title_shape.text).strip()
    if title_shape is not None
    else ""
)

python-pptx's shapes.title returns whatever placeholder sits at idx == 0, not
necessarily a title text placeholder. In this deck that slot holds a picture
placeholder, so python-pptx hands back a PlaceholderPicture, which has no
.text — only text-frame shapes do. Older decks (and decks built from custom
layouts) hit this regularly; the layout puts a logo or screenshot in the idx-0
placeholder.

Note this is not only about the title: a PlaceholderPicture title also means
the picture is silently lost today even if the AttributeError were swallowed,
because _render_shape skips any shape matching title_shape before it gets a
chance to be extracted as an image.

Expected

A picture in the title placeholder should not abort conversion of the deck. The
slide should fall back to the Slide N heading (the existing slide_title or f"Slide {slide_num}" path), and the picture should still be extracted as an
attachment like any other picture.

Suggested fix

Guard the title read on the shape actually having a text frame, and only treat
the title shape as "already rendered" when it contributed the heading:

title_shape: Any = slide.shapes.title
if title_shape is not None and getattr(title_shape, "has_text_frame", False):
    slide_title = str(title_shape.text).strip()
else:
    title_shape = None   # not a text title; let the shape loop render it normally
    slide_title = ""

Setting title_shape = None is what keeps the picture: _render_shape /
_render_text_frame both skip via _same_shape(shape, title_shape), and
_same_shape already tolerates None.

Test coverage

Add a fixture deck whose slide layout places a picture in the idx == 0
placeholder and assert:

1. conversion succeeds,
2. the heading is ## Slide 1,
3. the picture is written to the attachments dir and linked.

Environment

• mdd main (at time of report: 3f7f017)
• python-pptx as pinned in uv.lock
• source deck: a 2013 .pptx synced from SharePoint (path and deck title redacted)