Fix ConstraintCritic to enforce axis-wise velocity bounds for holonomic robots

[prajwalthakur]

Use axis-wise velocity constraints (vx, vy) in ConstraintCritic instead of magnitude-based bounds for holonomic robots.

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	closes #6042
Primary OS tested on	Ubuntu
Robotic platform tested on	/
Does this PR contain AI generated software?	No
Was this PR description generated by AI software?	Out of respect for maintainers, AI for human-to-human communications are banned

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Description of contribution in a few bullet points

• Updated ConstraintCritic score function for holonomic robots to use axis-wise velocity constraints (vx, vy) instead of magnitude-based checks.

Description of documentation updates required from your changes

Description of how this change was tested

• Added unit test for holonomic robot in CriticTests.ConstraintsCritic

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Future work that may be required in bullet points

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

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
..._mppi_controller/src/critics/constraint_critic.cpp	87.50%	1 Missing ⚠️

Files with missing lines	Coverage Δ
...nav2_mppi_controller/critics/constraint_critic.hpp	100.00% <100.00%> (ø)
..._mppi_controller/src/critics/constraint_critic.cpp	86.36% <87.50%> (+21.14%)	⬆️

... and 6 files with indirect coverage changes

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