Handle tiny SDF inertial data and improve warnings

[jslee02]

Resolves #436.

Summary

• clamp non-positive or tiny SDF masses to a minimum and warn
• keep inertia physically coherent when mass exists but inertia is missing; adjust tests accordingly
• skip macOS lint autofix and format SdfParser to satisfy CI

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable