Fix: Reset both sec and nanosec in time_from_start

[arnavk23]

Fixes issue #1708 by resetting both seconds and nanoseconds in time_from_start.

Previously, only seconds were reset with set__sec(0), but nanoseconds remained unchanged, causing incorrect time resets.

This patch sets both sec and nanosec to zero explicitly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.70%. Comparing base (1b92f30) to head (e9101b0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1709   +/-   ##
=======================================
  Coverage   85.69%   85.70%           
=======================================
  Files         123      123           
  Lines       11895    11896    +1     
  Branches     1014     1014           
=======================================
+ Hits        10194    10195    +1     
  Misses       1379     1379           
  Partials      322      322           

Flag	Coverage Δ
unittests	85.70% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	86.01% <100.00%> (+0.01%)	⬆️

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[christophfroehlich]

Thanks. I had a quick look to find a one-line assignment, but there is also none in rclcpp::duration.