Add approach_velocity_scaling_dist to dynamic parameter callback

[brammmieee]

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	N/A - Bug fix for missing dynamic parameter
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	Gazebo simulation, custom AMR platform
Does this PR contain AI generated software?	No
Was this PR description generated by AI software?	No

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Description of contribution in a few bullet points

• Fixed missing approach_velocity_scaling_dist parameter in dynamicParametersCallback function
• Parameter was previously declared and initialized but could not be updated at runtime
• Now consistent with all other dynamic parameters in the Regulated Pure Pursuit Controller

Description of documentation updates required from your changes

• No documentation updates required - this fixes existing documented functionality
• The parameter is already documented, this just makes dynamic reconfiguration work as expected

Description of how this change was tested

• Verified parameter can be changed via rqt_reconfigure during runtime
• Confirmed parameter updates take effect immediately without node restart
• Tested that velocity scaling behavior responds correctly to parameter changes
• All other existing parameters continue to work as expected
• Performed linting validation using colcon test

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Future work that may be required in bullet points

• None - this is a standalone bug fix that makes the parameter work as originally intended

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists
• Should this be backported to current distributions? If so, tag with backport-humble, backport-iron.

[mini-1235]

@brammmieee Please check the build error: https://app.circleci.com/pipelines/github/ros-navigation/navigation2/16812/workflows/d927e01c-8538-4765-8795-10813d7c8c85/jobs/49285

Also, please sign with DCO

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
..._pure_pursuit_controller/src/parameter_handler.cpp	0.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
..._pure_pursuit_controller/src/parameter_handler.cpp	92.09% <0.00%> (-0.74%)	⬇️

... and 13 files with indirect coverage changes

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[SteveMacenski]

Please add to the tests so we exercise the code for coverage https://github.com/ros-navigation/navigation2/blob/main/nav2_regulated_pure_pursuit_controller/test/test_regulated_pp.cpp#L425-L521. Should be a 2 line change.