Fix: Validate external forces to prevent NaN/Inf crash (DART 6.16)

[jslee02]

Summary

• Add NaN/Inf validation to BodyNode::addExtForce, setExtForce, addExtTorque, setExtTorque
• Add NaN/Inf validation to PointMass::addExtForce
• Invalid values are logged as warnings and ignored to prevent simulation crashes

Related Issue

Addresses gz-physics#844 - Crash in BodyNode::updateBiasForce when Gazebo Hydrodynamics plugin passes NaN parameters (e.g., <xDotU>NaN</xDotU>).

Changes

File	Change
dart/dynamics/BodyNode.cpp	Validate force/torque vectors before applying
dart/dynamics/PointMass.cpp	Validate force vector before applying
tests/unit/dynamics/test_BodyNodeExternalForce.cpp	9 unit tests for NaN/Inf handling
tests/unit/dynamics/CMakeLists.txt	New test target
tests/unit/CMakeLists.txt	Add dynamics subdirectory

Testing

All 9 new unit tests pass locally:

• AddExtForceWithNaN
• AddExtForceWithInf
• AddExtForceWithNaNOffset
• SetExtForceWithNaN
• AddExtTorqueWithNaN
• AddExtTorqueWithInf
• SetExtTorqueWithNaN
• ValidForceApplication
• GzPhysics844Scenario

[codecov]

Codecov Report

❌ Patch coverage is 74.13793% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.85%. Comparing base (991315e) to head (2ad582d).
⚠️ Report is 5 commits behind head on release-6.16.

Files with missing lines	Patch %	Lines
dart/dynamics/PointMass.cpp	0.00%	8 Missing ⚠️
dart/dynamics/BodyNode.cpp	86.00%	7 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-6.16    #2443      +/-   ##
================================================
+ Coverage         58.74%   58.85%   +0.11%     
================================================
  Files               385      385              
  Lines             32194    32241      +47     
  Branches           3884     3896      +12     
================================================
+ Hits              18911    18977      +66     
+ Misses            13283    13264      -19     

Flag	Coverage Δ
unittests	58.85% <74.13%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
dart/dynamics/BodyNode.hpp	50.00% <ø> (ø)
dart/dynamics/PointMass.hpp	100.00% <ø> (ø)
dart/dynamics/BodyNode.cpp	80.10% <86.00%> (+0.65%)	⬆️
dart/dynamics/PointMass.cpp	48.35% <0.00%> (-0.43%)	⬇️

... and 5 files with indirect coverage changes

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[jslee02]

@codex review

This PR has been rebased on release-6.16. The validation approach using DART_WARN with early return is appropriate for this use case (handling NaN/Inf external forces from hydrodynamics parameters in gz-physics).

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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