Include a reason when a task has never started

[chkeita]

Summary of the Pull Request

• Added a reason field to MarkStopping
• Added a new ErrorCode TASK_CANCELLED to differentiate cancellation from failure.
• If a task is cancelled all dependent task are cancelled

closes #3147

[codecov-commenter]

Codecov Report

Merging #3148 (0960f40) into main (5d8f245) will increase coverage by 0.00%.
The diff coverage is 63.63%.

@@           Coverage Diff           @@
##             main    #3148   +/-   ##
=======================================
  Coverage   31.59%   31.59%           
=======================================
  Files         308      308           
  Lines       36563    36564    +1     
=======================================
+ Hits        11552    11553    +1     
  Misses      25011    25011           

Impacted Files	Coverage Δ
src/ApiService/ApiService/Functions/Tasks.cs	22.22% <0.00%> (ø)
src/ApiService/ApiService/Functions/TimerTasks.cs	0.00% <0.00%> (ø)
src/ApiService/ApiService/OneFuzzTypes/Enums.cs	32.39% <ø> (ø)
.../ApiService/ApiService/onefuzzlib/JobOperations.cs	10.34% <0.00%> (ø)
...ApiService/ApiService/onefuzzlib/TaskOperations.cs	31.09% <85.71%> (+0.29%)	⬆️
src/ApiService/ApiService/Functions/AgentEvents.cs	55.60% <100.00%> (ø)