fix: raise a clear error when concatenating along a scalar coord

[maxrjones]

@hrodmn hit the obscure missing chunk manager error today, this time because xarray tried to load data when generating indexes for concatenating along a dimension that has scalar coordinates. This PR raises a clear error with informative guidance (load the variable).

This is a distinct from existing chunk manager issues so it doesn't close anything, but is related to #114.

Acceptance criteria:

• Closes #xxxx
• Tests added
• Tests passing
• No test coverage regression
• Full type hint coverage
• Changes are documented in docs/releases.md
• New functions/methods are listed in an appropriate *.md file under docs/api
• New functionality has documentation

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.16%. Comparing base (b693e0d) to head (f90ef7a).

Files with missing lines	Patch %	Lines
virtualizarr/xarray.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1015      +/-   ##
==========================================
+ Coverage   89.97%   90.16%   +0.18%     
==========================================
  Files          36       36              
  Lines        2224     2236      +12     
==========================================
+ Hits         2001     2016      +15     
+ Misses        223      220       -3     

Files with missing lines	Coverage Δ
virtualizarr/xarray.py	90.00% <91.66%> (+3.55%)	⬆️

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[TomNicholas]

Thanks, let's de-claude the tests before merging.

[TomNicholas]

I think we should delete all the other tests from this PR except the first part of this one. Every other test here either tests a private internal, or tests a behaviour that should already be tested.