fix: DH-21407: Dehydrate/hydrate pending data correctly (#2606)

[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 48.57%. Comparing base (549e4e9) to head (65bfd28).
⚠️ Report is 3 commits behind head on release/v0.85.

Additional details and impacted files

@@                Coverage Diff                @@
##           release/v0.85    #2609      +/-   ##
=================================================
+ Coverage          48.44%   48.57%   +0.12%     
=================================================
  Files                712      712              
  Lines              39761    39789      +28     
  Branches           10121    10129       +8     
=================================================
+ Hits               19263    19326      +63     
+ Misses             20485    20406      -79     
- Partials              13       57      +44     

Flag	Coverage Δ
unit	48.57% <100.00%> (+0.12%)	⬆️

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