refactor(motion_velocity_planner): simplify insert stop point function

[maxime-clem]

Description

Minor change to simplify the function to add stop points to the motion velocity planner trajectory.

Related links

Parent Issue:

• Link

How was this PR tested?

Notes for reviewers

None.

Interface changes

None.

Effects on system behavior

None.

[github-actions]

Thank you for contributing to the Autoware project!

🚧 If your pull request is in progress, switch it to draft mode.

Please ensure:

• You've checked our contribution guidelines.
• Your PR follows our pull request guidelines.
• All required CI checks pass before marking the PR ready for review.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (4cb18f5) to head (23985a1).
Report is 174 commits behind head on main.

Files with missing lines	Patch %	Lines
...nner/autoware_motion_velocity_planner/src/node.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #375       +/-   ##
==========================================
- Coverage   78.75%   0.00%   -78.76%     
==========================================
  Files          11     101       +90     
  Lines         193    7079     +6886     
  Branches       73    1279     +1206     
==========================================
- Hits          152       0      -152     
- Misses         11    7079     +7068     
+ Partials       30       0       -30     

Flag	Coverage Δ
differential	0.00% <0.00%> (?)
total	?

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mitsudome-r]

Could you explain how this modification is tested?