Ensure ChatMessage internals are visible to Panel

[philippjfr]

Fixes #6300

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1a0a670) 82.50% compared to head (9f4e222) 82.65%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
panel/chat/message.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6304      +/-   ##
==========================================
+ Coverage   82.50%   82.65%   +0.14%     
==========================================
  Files         305      305              
  Lines       45419    45421       +2     
==========================================
+ Hits        37475    37542      +67     
+ Misses       7944     7879      -65     

Flag	Coverage Δ
ui-tests	38.55% <50.00%> (+0.21%)	⬆️
unitexamples-tests	71.71% <50.00%> (+0.08%)	⬆️

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