fix: restore Polygon cutline schema for Shapely 2

[MJohnson459]

Fix split-merge cutline GPKG writes failing under Shapely 2 + Fiona 1.9+.

This came up with running OATS brighton_split_merge test:

Traceback (most recent call last):
  File "/code/run.py", line 65, in <module>
    retcode = app.execute()
              ^^^^^^^^^^^^^
  File "/code/stages/odm_app.py", line 118, in execute
    raise e
  File "/code/stages/odm_app.py", line 82, in execute
    self.first_stage.run()
  File "/code/opendm/types.py", line 465, in run
    self.next_stage.run(outputs)
  File "/code/opendm/types.py", line 465, in run
    self.next_stage.run(outputs)
  File "/code/opendm/types.py", line 465, in run
    self.next_stage.run(outputs)
  [Previous line repeated 7 more times]
  File "/code/opendm/types.py", line 444, in run
    self.process(self.args, outputs)
  File "/code/stages/odm_orthophoto.py", line 122, in process
    compute_cutline(tree.odm_orthophoto_tif, 
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/code/opendm/cutline.py", line 170, in compute_cutline
    sink.write({
  File "/code/.pixi/envs/prod/lib/python3.12/site-packages/fiona/collection.py", line 551, in write
    self.writerecords([record])
  File "/code/.pixi/envs/prod/lib/python3.12/site-packages/fiona/collection.py", line 541, in writerecords
    self.session.writerecs(records, self)
  File "fiona/ogrext.pyx", line 1660, in fiona.ogrext.WritingSession.writerecs
fiona.errors.GeometryTypeValidationError: Record's geometry type does not match collection schema's geometry type: 'Polygon' != 'MultiPolygon'

The root cause is related to two dependency bumps we did in #1958 - Shapely 2 no longer has __getitem__ for MultiPolygon and so unary_union results were written whole instead of picking the largest polygon. Commit 75a1e1f only fixed one half of that mismatch by changing the schema to MultiPolygon but Fiona 1.9+ still rejects Polygon records against a MultiPolygon schema.

Note #1958 testing originally hit the opposite mismatch (MultiPolygon record vs Polygon schema) before 75a1e1f; this PR fixes both directions by fixing Shapely unpacking and restoring the Polygon schema.

[DodgySpaniard]

Usually, I would add some comments and/or "ASCII" art to visualise what is going on.

Right now the tests are not super clear to me.

[MJohnson459]

Added a few more comments to explain what was going on 👍