Unify equality helpers and silence -Wfloat-equal

[jslee02]

Summary

• add valueEqual/isEqual/isApprox/isZero templates for scalars and Eigen types; handle +/-0 and NaNs
• switch GenericJoint macro to valueEqual to avoid -Wfloat-equal in consumers
• update GTest equals helper to share logic and added coverage tests

Testing

• pixi run lint
• CI pending

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.63%. Comparing base (aca67ea) to head (6b420f1).

Files with missing lines	Patch %	Lines
dart/math/Helpers.hpp	84.21%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2225      +/-   ##
==========================================
+ Coverage   62.62%   62.63%   +0.01%     
==========================================
  Files         361      361              
  Lines       32794    32813      +19     
  Branches     4314     4318       +4     
==========================================
+ Hits        20536    20553      +17     
- Misses      12258    12260       +2     

Flag	Coverage Δ
unittests	62.63% <84.21%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
dart/dynamics/detail/GenericJoint.hpp	79.14% <ø> (ø)
dart/math/Helpers.hpp	78.72% <84.21%> (+3.72%)	⬆️

... and 1 file with indirect coverage changes

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