Fix: Reset both sec and nanosec in time_from_start (backport #1709)

[mergify]

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.

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This is an automatic backport of pull request #1709 done by Mergify.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.86%. Comparing base (080299a) to head (0defc6c).
Report is 1 commits behind head on jazzy.

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy    #1711   +/-   ##
=======================================
  Coverage   84.86%   84.86%           
=======================================
  Files         127      127           
  Lines       12146    12147    +1     
  Branches     1036     1036           
=======================================
+ Hits        10308    10309    +1     
  Misses       1499     1499           
  Partials      339      339           

Flag	Coverage Δ
unittests	84.86% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	85.65% <100.00%> (+0.01%)	⬆️

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