fix: DH-21407: Dehydrate/hydrate pending data correctly

[mofojed]

• We were tryin to dehydrate the CellData object instead of the value itself
• Just dehydrate the value itself
• Fix the tests
• This seems to have been broken for a long time, and only now it's being caught when attempting to persist deephaven.ui data

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.96%. Comparing base (ff7b424) to head (11ca029).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2606      +/-   ##
==========================================
+ Coverage   45.71%   45.96%   +0.25%     
==========================================
  Files         771      771              
  Lines       43652    43652              
  Branches    11231    11231              
==========================================
+ Hits        19954    20065     +111     
+ Misses      23653    23570      -83     
+ Partials       45       17      -28     

Flag	Coverage Δ
unit	45.96% <100.00%> (+0.25%)	⬆️

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