BUG: raise informative error for MultiPolygon in corner-based shape metrics (#739)

[gaoflow]

Fixes #739.

Problem

corners, squareness and centroid_corner_distance crash or silently misbehave on MultiPolygon input. With include_interiors=False they use GeoSeries.exterior, which is undefined (None) for MultiPolygons. get_coordinates() then drops those rows, so the grouped result is shorter than the input index:

• corners / squareness → ValueError: Length mismatch (cryptic)
• centroid_corner_distance → silently returns all-NaN (with RuntimeWarning: Mean of empty slice)

Fix

Per the discussion on #739, this takes the informative-error path rather than the design-gated full-MultiPolygon-support path. A shared _check_no_multipolygons guard raises an actionable ValueError (pointing to geometry.explode(ignore_index=True) or include_interiors=True) in the three affected functions. This turns a cryptic crash / silent-NaN into a clear message. Happy to extend to full MultiPolygon support instead if that's preferred.

Tests

Added a parametrized test_corner_metrics_multipolygon over the three metrics: asserts the informative ValueError, that include_interiors=True still works, and that the .explode() workaround works. Without the fix all three fail (two with the length-mismatch, one with the silent NaN); with it test_shape.py passes (20).

Disclosure: I used AI assistance (Claude) to help locate and draft this fix, under my direction and review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.3%. Comparing base (4037c70) to head (2d906f9).
⚠️ Report is 162 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #763     +/-   ##
=======================================
+ Coverage   97.4%   98.3%   +0.9%     
=======================================
  Files         26      28      +2     
  Lines       4328    4573    +245     
=======================================
+ Hits        4214    4495    +281     
+ Misses       114      78     -36     

Files with missing lines	Coverage Δ
momepy/shape.py	99.2% <100.0%> (+7.6%)	⬆️
momepy/tests/test_shape.py	100.0% <100.0%> (ø)

... and 17 files with indirect coverage changes

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[martinfleis]

Thanks! Certainly useful.