Refactor ChatFeed post hook

[ahuang11]

As I was building out logs for Lumen, I noticed that depending on message = self._chat_log.objects[-1] was insufficient due to race conditions (i.e. needed a direct ref to the corresponding message for accuracy).

Also, I noticed that the post hook was prematurely being called before ChatStep finished streaming. Now, it depends on its status changing to success or failed, before triggering the post hook.

Semi-breaking change is stream no longer triggers post hook by default.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.06%. Comparing base (8f9cc0f) to head (f0f2ff7).

Files with missing lines	Patch %	Lines
panel/chat/feed.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7722      +/-   ##
==========================================
+ Coverage   87.04%   87.06%   +0.02%     
==========================================
  Files         346      346              
  Lines       52711    52743      +32     
==========================================
+ Hits        45883    45922      +39     
+ Misses       6828     6821       -7     

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