Reorganize tests into logical subdirectories

[jeongseok-meta]

• Group integration tests by category: collision, constraint, dynamics, io, optimization, simulation, utils
• Organize unit tests: collision, constraint, dynamics, lcpsolver
• Rename regression tests from Issue#### to descriptive names
• Consolidate test helpers into tests/helpers/
• Remove deprecated tests
• Update CMake configuration

---

Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes
• Document new methods and classes
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve warnings

Before merging

• Set milestone
• Update CHANGELOG.md
• Add unit tests
• Add Python bindings (dartpy) if applicable

[chatgpt-codex-connector]

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[jslee02]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.47%. Comparing base (1e4ea46) to head (15b03b7).
⚠️ Report is 396 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2071      +/-   ##
==========================================
- Coverage   59.06%   58.47%   -0.59%     
==========================================
  Files         386      386              
  Lines       32475    32475              
  Branches     3969     3964       -5     
==========================================
- Hits        19182    18991     -191     
- Misses      13293    13484     +191     

Flag	Coverage Δ
unittests	58.47% <ø> (-0.59%)	⬇️

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see 2 files with indirect coverage changes

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