fix: round-trip timedelta cron offset

[danfimov]

Closes #605

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.37%. Comparing base (1bcb01a) to head (500720f).

Files with missing lines	Patch %	Lines
taskiq/scheduler/scheduled_task/validators.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #679      +/-   ##
==========================================
+ Coverage   85.33%   85.37%   +0.03%     
==========================================
  Files          68       68              
  Lines        2646     2660      +14     
==========================================
+ Hits         2258     2271      +13     
- Misses        388      389       +1     

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