fix: Prevent scheduling background events less than 1 second in the future

[FrederikBolding]

This PR fixes an issue with scheduling background events using durations. Since we were truncating durations to remove milliseconds we would effectively allow scheduling events less than 1 second into the future.

To work around this, durations are now allowed to be a minimum of 1 second and are untruncated. The execution date will still be truncated when the Snap requests it to prevent leakage of millisecond precision.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.12%. Comparing base (4acb5c3) to head (e3b606e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3414   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files         399      399           
  Lines       10961    10967    +6     
  Branches     1725     1727    +2     
=======================================
+ Hits        10755    10761    +6     
  Misses        206      206           

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