Backport PR #3994 on branch 1.12.x (fix: raise ValueError when combat batch has fewer than 2 cells)

[meeseeksmachine]

Backport PR #3994: fix: raise ValueError when combat batch has fewer than 2 cells

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.28%. Comparing base (fec6eb4) to head (9312d97).
⚠️ Report is 1 commits behind head on 1.12.x.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           1.12.x    #4001   +/-   ##
=======================================
  Coverage   78.28%   78.28%           
=======================================
  Files         117      117           
  Lines       12635    12639    +4     
=======================================
+ Hits         9891     9895    +4     
  Misses       2744     2744           

Flag	Coverage Δ
hatch-test.low-vers	77.57% <100.00%> (+<0.01%)	⬆️
hatch-test.pre	77.23% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/scanpy/preprocessing/_combat.py	88.80% <100.00%> (+0.34%)	⬆️