Fix typing errors

[maxrjones]

Addresses this failing workflow - https://github.com/zarr-developers/VirtualiZarr/actions/runs/20701631805

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.41%. Comparing base (cf8e0db) to head (c3c49f0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #847      +/-   ##
==========================================
- Coverage   88.51%   88.41%   -0.10%     
==========================================
  Files          35       35              
  Lines        2003     2003              
==========================================
- Hits         1773     1771       -2     
- Misses        230      232       +2     

Files with missing lines	Coverage Δ
virtualizarr/manifests/array.py	81.72% <100.00%> (ø)
virtualizarr/manifests/array_api.py	96.87% <ø> (ø)

... and 1 file with indirect coverage changes

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[maxrjones]

docs failure is addressed separately in #846

[TomNicholas]

I presume this is because of numpy's type promotion rules.