[6.16] Fix pybind11 detection and assert includes

[jslee02]

Summary

• Disable Boost's embedded gdb scripts when compiling with GCC so inline asm helpers stop breaking -Werror builds.
• Make CMake look up pybind11 from the active Python environment to keep dartpy wheels (e.g., CPython 3.13 on macOS) building consistently.
• Keep the <cassert> includes in the core/common codepaths so clang-format remains stable in CI, while using DART_ASSERT throughout the optimizer/common code instead of raw assert.

Testing

• pixi run test-all
• pixi run check-lint-cpp

---

Before creating a pull request

• Format code using pixi run lint and verify with pixi run check-lint
• 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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.67%. Comparing base (a2f18c0) to head (178d994).
⚠️ Report is 54 commits behind head on release-6.16.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-6.16    #2118   +/-   ##
=============================================
  Coverage         58.67%   58.67%           
=============================================
  Files               384      384           
  Lines             32059    32059           
  Branches           3870     3870           
=============================================
  Hits              18810    18810           
  Misses            13249    13249           

Flag	Coverage Δ
unittests	58.67% <ø> (ø)

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

Files with missing lines	Coverage Δ
dart/constraint/BoxedLcpConstraintSolver.cpp	50.00% <ø> (ø)
dart/constraint/LCPSolver.cpp	0.00% <ø> (ø)

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