Use the new declare_or_get_parameter API for nav2_graceful_controller

[Hobanghann]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#5299
Primary OS tested on	Ubuntu
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No
Was this PR description generated by AI software?	N/A

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Description of contribution in a few bullet points

Migrated the get_parameter and declare_parameter fields to use the new declare_or_get_parameter API for nav2_graceful_controller.

Description of documentation updates required from your changes

N/A

Description of how this change was tested

Performed linting validation using pre-commit run --all and testing locally using colcon test

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Future work that may be required in bullet points

N/A

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-*.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
nav2_graceful_controller/src/parameter_handler.cpp	98.31% <100.00%> (-0.29%)	⬇️

... and 2 files with indirect coverage changes

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[mini-1235]

LGTM