[DOP-20962] Add scheduler integration test

[IlyasDevelopment]

Change Summary

Add integration test for scheduler. The full cycle of the shedualer operation is checked:

1. Task initialization in the database
2. Task scheduling via APScheduler
3. Sending the task to the Celery queue
4. Task execution by Celery worker
5. Updating the execution status
6. Checking in the database that the task was completed successfully

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.63%. Comparing base (87044fb) to head (532a065).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #118      +/-   ##
===========================================
- Coverage    93.03%   87.63%   -5.41%     
===========================================
  Files          129      130       +1     
  Lines         3089     3090       +1     
  Branches       241      241              
===========================================
- Hits          2874     2708     -166     
- Misses         157      324     +167     
  Partials        58       58              

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