Raise a clear error when a da_nhoods design has no residual degrees of freedom

[Zethson]

Closes #1075.

With one sample per condition, edgeR's estimateDisp sets the dispersion to NA and glmQLFit then fails with missing value where TRUE/FALSE needed, which gives no hint about the actual problem.
da_nhoods now checks that the design leaves residual degrees of freedom and raises a ValueError pointing at the missing replication, for all three solvers.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.47%. Comparing base (0c429c5) to head (974e7c1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pertpy/tools/_milo.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1077   +/-   ##
=======================================
  Coverage   79.46%   79.47%           
=======================================
  Files          53       53           
  Lines        7545     7556   +11     
=======================================
+ Hits         5996     6005    +9     
- Misses       1549     1551    +2     

Files with missing lines	Coverage Δ
src/pertpy/tools/_milo.py	78.42% <81.81%> (+0.17%)	⬆️

... and 1 file with indirect coverage changes

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