fix(taskgroup): failed group tasks not counted

[alitto]

This pull request includes changes to the group.go file to improve task submission handling and adds new test cases to ensure the functionality of task group metrics and handling of cancelled contexts.

Improvements to task submission:

• Changed the submit function to use g.pool.dispatcher.Write for task execution, which returns an error if the context is cancelled or an error occurs during task invocation. (group.go, group.goL107-R115)
• Added a return statement to ensure the error is properly returned after task completion. (group.go, group.goR125-R126)

New test cases:

• Added TestTaskGroupMetrics to verify the correct reporting of submitted, successful, and failed tasks. (group_test.go, group_test.goR326-R371)
• Added TestTaskGroupMetricsWithCancelledContext to test the behavior of task groups when the context is cancelled, ensuring accurate metrics for successful and failed tasks. (group_test.go, group_test.goR326-R371)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.11%. Comparing base (543ed3a) to head (688b02c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   96.10%   96.11%           
=======================================
  Files          14       14           
  Lines         847      849    +2     
=======================================
+ Hits          814      816    +2     
  Misses         30       30           
  Partials        3        3           

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