Fix ChatFeed / Interface tests and async generator placeholders

[ahuang11]

Ensures that the tests actually run and raises errors by setting callback_exception = "raise" rather than having the errors piped to the chat logs

This resulted in finding a lot of tests failing, but most of them were just tests not written properly (e.g. not using wait_until to assert)

The only bug I found was a result of #6105, where async generator functions execution time was based on placeholder_threshold, i.e. await _schedule_placeholder had to run in completion, before the actual callback started executing.

This was remedied by wrapping _serialize_response (where the iteration of the generator was happening) alongside the actual callback so that when checking task.done(), it checks the execution of both of these.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c8f4e08) 82.68% compared to head (cbcf154) 84.19%.
Report is 11 commits behind head on main.

Files	Patch %	Lines
panel/tests/chat/test_feed.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6245      +/-   ##
==========================================
+ Coverage   82.68%   84.19%   +1.51%     
==========================================
  Files         301      301              
  Lines       45099    45137      +38     
==========================================
+ Hits        37288    38004     +716     
+ Misses       7811     7133     -678     

Flag	Coverage Δ
ui-tests	40.70% <10.44%> (+2.22%)	⬆️
unitexamples-tests	71.87% <97.01%> (+0.04%)	⬆️

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[ahuang11]

I manually tested this branch on a few examples:

Screen.Recording.2024-01-19.at.11.05.48.AM.mov

Screen.Recording.2024-01-19.at.11.06.34.AM.mov

Screen.Recording.2024-01-19.at.11.08.31.AM.mov