fix(motion_velocity_smoother): insert the point when applying external velocity limit

[TomohitoAndo]

Description

In the current implementation, when applying the external velocity limit, the velocity of the points on the trajectory are being overwritten. However, this implementation has an issue where the inserting position of the external velocity limit is further away than expected and ego vehicle sometimes can't stop at the desired position (see the video below).
In this PR, I have resolved this problem by inserting the point of the external velocity limit.

external_vel_limit_cannot_stop.mp4

Before

After

Tests performed

I confirmed this works with simple planning simulator.

external_vel_limit_psim.mp4

Effects on system behavior

Not applicable.

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[codecov]

Codecov Report

Patch coverage: 63.63% and no project coverage change.

Comparison is base (27fa619) 13.80% compared to head (87008dc) 13.81%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3494   +/-   ##
=======================================
  Coverage   13.80%   13.81%           
=======================================
  Files        1391     1391           
  Lines       97408    97411    +3     
  Branches    28829    28827    -2     
=======================================
+ Hits        13451    13454    +3     
- Misses      69501    69502    +1     
+ Partials    14456    14455    -1     

Flag	Coverage Δ		*Carryforward flag
differential	6.56% <63.63%> (?)
total	13.80% <ø> (-0.01%)	⬇️	Carriedforward from 7d04f0b

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
...ity_smoother/src/motion_velocity_smoother_node.cpp	23.31% <63.63%> (+0.44%)	⬆️

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