Fix generator placeholder and optimize updates

[ahuang11]

In generator tasks, the condition while not task.done() and num_entries == len(self._chat_log): leads to tasks completing almost instantly, as generators are lazily evaluated. Consequently, relying on task.done() becomes unreliable.

When using submit_task(generator), the state.FiNISHED is quickly reached because, technically, the generator is lazily evaluated and returned. To illustrate, the generator is considered "finished" in a mere 1.33 microseconds, as depicted in the image below:

So it's not reliable to depend on task.done() Instead, we should use callback_state.IDLE to track business (when re-reading this, I was confused and realized, business -> busy-ness)

Also, this updates how the placeholder text gets updated.

Addresses
https://discourse.holoviz.org/t/spinning-icon-effect-that-appears-when-the-user-sends-a-message-in-chatinterface/6582/10?u=ahuang11

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (7f25eb1) 84.67% compared to head (7c97bfa) 84.65%.

Files	Patch %	Lines
panel/chat/feed.py	50.00%	2 Missing ⚠️
panel/tests/chat/test_feed.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6105      +/-   ##
==========================================
- Coverage   84.67%   84.65%   -0.02%     
==========================================
  Files         296      296              
  Lines       44100    44105       +5     
==========================================
- Hits        37341    37338       -3     
- Misses       6759     6767       +8     

Flag	Coverage Δ
ui-tests	40.69% <6.66%> (-0.01%)	⬇️
unitexamples-tests	72.66% <80.00%> (-0.02%)	⬇️

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