Allow background exceptions to be fatal

[jackgerrits]

Closes #4904

Does not change default behavior in core.

In agentchat, this change will mean that exceptions that used to be ignored and result in bugs like the group chat stopping are now reported out to the user application.

[jackgerrits]

Based on my testing, I believe it is currently invalid to let an agentchat runtime ignore background exceptions.

What happens is, an agent is invoked to produce its response. Instead of the agent publishing its response, it raises an exception. The runtime catches this, logs it, and continues on. However, since no result was published the runtime now becomes idle and the runtime terminates. This causes early termination of the team, but it also doesn't propagate the error as it was ignored is the background.

Based on this, I think the current design of agentchat does not allow for automatic recovery from exception and we should in fact require these exceptions to be surfaced.

@ekzhu thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.67%. Comparing base (dc55ec9) to head (e932c60).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5716      +/-   ##
==========================================
- Coverage   74.91%   71.67%   -3.25%     
==========================================
  Files         175      115      -60     
  Lines       11137     8225    -2912     
==========================================
- Hits         8343     5895    -2448     
+ Misses       2794     2330     -464     

Flag	Coverage Δ
unittests	71.67% <100.00%> (-3.25%)	⬇️

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[jackgerrits]

CC @lokitoth for dotnet

[ekzhu]

Agree. I think the exceptions should be surfaced to the application.