Fix dynamic chat reaction icons

[ahuang11]

Follow up on https://github.com/holoviz/panel/pull/6807/files, properly implementing the fix

• inserting the reactive expression directly (which indeed means it’ll be nested inside a ReactiveExpr pane which creates a layout)
• handling updates to the reactive_icons correctly, so stylesheets, the link and show_reaction_icons are not reapplied.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 40.09%. Comparing base (b3fcdad) to head (dbbdd64).
Report is 10 commits behind head on main.

Files	Patch %	Lines
panel/tests/chat/test_message.py	18.18%	9 Missing ⚠️
panel/chat/message.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6815       +/-   ##
===========================================
- Coverage   81.54%   40.09%   -41.46%     
===========================================
  Files         314      314               
  Lines       46453    46575      +122     
===========================================
- Hits        37879    18672    -19207     
- Misses       8574    27903    +19329     

Flag	Coverage Δ
ui-tests	40.09% <50.00%> (+2.61%)	⬆️
unitexamples-tests	?

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