Fix DART crash caused by non-positive definite spatial inertia

[XINJIANGMO]

🦟 Bug fix

Fixes #3246

Summary

Previously, if a model contained invalid inertial properties (specifically, a non-positive definite spatial inertia matrix, often caused by incorrect fluid_added_mass coefficients), the invalid matrix was passed directly to DART, triggering an assertion failure (Assertion !math::isNan(partialAcceleration) failed) and causing the server to abort.

This patch adds a validation step in Physics.cc (both in CreateModelEntities and CreateLinkEntities) using Eigen::LLT decomposition to ensure the spatial inertia matrix is positive definite before passing it to the physics engine.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)