Render constant-value channels as mid-grey with a warning

[timtreis]

Summary

• A channel where all pixel values are identical causes Normalize to compute 0/0, silently producing an all-black composite with no error or warning
• Before normalization, check if the channel is constant (min == max) and no explicit vmin/vmax was provided; if so, substitute 0.5 and emit a logger.warning
• This makes the channel visible (mid-grey) and informs the user their data has no contrast in that channel

Fixes #598.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.41%. Comparing base (9afec7e) to head (e92b734).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #636   +/-   ##
=======================================
  Coverage   76.40%   76.41%           
=======================================
  Files          11       11           
  Lines        3230     3235    +5     
  Branches      758      759    +1     
=======================================
+ Hits         2468     2472    +4     
  Misses        464      464           
- Partials      298      299    +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	86.59% <100.00%> (+0.10%)	⬆️

... and 1 file with indirect coverage changes

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