Add C++ unit tests for tensor_momentum library

[cstollmeta]

Summary:
Adds 42 GoogleTest cases in cpp_test/tensor_momentum_test.cpp covering core tensor operations from the tensor_momentum library. The tests cover four of the ten untested source files that have standalone C++ APIs testable without complex Character object setup:

• tensor_quaternion.h (15 tests): quaternion identity, check validation, split, normalize, conjugate, inverse, multiply (identity and batch), rotate vector, Euler-to-quaternion round-trip, quaternion-to-rotation-matrix round-trip, rotation-matrix-to-quaternion round-trip, blend quaternions, weight normalization
• tensor_transforms.h (14 tests): identity skeleton state, split skeleton state, quaternion/translation/scale to skeleton state, skeleton state to transforms (identity and translation), multiply skeleton states, transform points (identity, translation, scale, invalid input), inverse skeleton states (with and without rotation), blend skeleton states
• tensor_kd_tree.h (4 tests): find closest points (exact match, nearest neighbor, max distance filtering), find closest points on mesh (triangle projection)
• tensor_skinning.h (3 tests): compute vertex normals (simple triangle, invalid input validation, normalization check)

Tests focus on edge cases, numerical precision, and input validation — areas where Python integration tests typically lack coverage. The remaining 6 source files (tensor_blend_shape, tensor_parameter_transform, tensor_skeleton_state, tensor_mppca, tensor_joint_parameters_to_positions, tensor_momentum_utility) require Character/ParameterTransform objects from the momentum library and are better suited for a follow-up with appropriate test fixtures.

Reviewed By: yutingye

Differential Revision: D95466235

[meta-codesync]

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