fix: DH-18648: Ensure grid panel doesn't crash when Pandas query restarted

[mofojed]

• Taking part of https://github.com/deephaven-ent/iris/pull/2572, but only need to reset the model on disconnect in G+
• The model is refetched the query is restarted
• Tested against dev-gplus, created a query there with some pandas data frames

[codecov]

Codecov Report

❌ Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.59%. Comparing base (4c2561c) to head (021b8a2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../dashboard-core-plugins/src/panels/PandasPanel.tsx	0.00%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2528      +/-   ##
==========================================
- Coverage   44.61%   44.59%   -0.03%     
==========================================
  Files         764      764              
  Lines       42800    42825      +25     
  Branches    10964    10769     -195     
==========================================
  Hits        19096    19096              
- Misses      23693    23718      +25     
  Partials       11       11              

Flag	Coverage Δ
unit	44.59% <0.00%> (-0.03%)	⬇️

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