Add Student-t covariance scaling helper

[FlorianPfaff]

Summary

• add student_t_covariance_scale(...) next to the existing Huber covariance scaling helper
• export the helper from pyrecest.filters
• add focused tests for formula agreement, inlier clamping, vectorized NIS inputs, and argument validation

Notes

• this is the minimal PyRecEst backport of the reusable Student-t covariance scaling used by the RaFT-UAV robust-update work
• no RaFT-UAV-specific tracking, CLI, or experiment code is included

Tests

• not run here; container DNS cannot reach GitHub to install/clone dependencies
• intended targeted test: python -m pytest tests/test_linear_gaussian_robust.py

[github-actions]

✅MegaLinter analysis: Success

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