fix: scalar variable manifests get shape () not (1,) from kerchunk refs

[TomNicholas]

Closes #966

Summary

• Kerchunk uses v2-style chunk key "0" for scalar variables, which get_chunk_grid_shape interprets as 1D shape (1,) instead of scalar (). This causes ManifestStore.get_entry to return an ndarray instead of a string when resolving scalar chunk paths, crashing urlparse.
• Pass the chunk grid shape derived from array metadata into ChunkManifest, so scalar variables get the correct () shape.
• Handle 0-d array indexing in ChunkManifest.__init__ when shape is ().

Test plan

• Added test_scalar_variable_manifest_shape — verifies scalar variable from kerchunk refs produces manifest with shape_chunk_grid == ()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (ce729ca) to head (d1cd415).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #965   +/-   ##
=======================================
  Coverage   89.30%   89.31%           
=======================================
  Files          33       33           
  Lines        2039     2031    -8     
=======================================
- Hits         1821     1814    -7     
+ Misses        218      217    -1     

Files with missing lines	Coverage Δ
virtualizarr/manifests/manifest.py	86.18% <ø> (ø)
virtualizarr/parsers/kerchunk/translator.py	81.08% <100.00%> (-0.44%)	⬇️

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