Fix cubic spline evaluation at the final knot

[YaoxinHuang]

Reference issue

No existing issue found for this endpoint failure.

What does this implement/fix?

Evaluating a cubic spline at its final input knot currently raises IndexError: bisect selects index nx - 1, but the polynomial coefficients only contain nx - 1 segments. This affects 1D position and derivatives, as well as 2D endpoint position, heading and curvature.

Clamp the selected index to the final segment so the endpoint is evaluated using that polynomial. Add regressions for two-point and curved natural splines against SciPy, plus a 2D endpoint check.

Additional information

Prepared with OpenAI Codex. Local validation on Windows / Python 3.13.13: the 9 new regressions fail on the original code and pass with this fix; 3 path-tracking tests and 5 Frenet scenarios pass. Ruff and the CubicSpline mypy check pass. The complete suite was not run because the pinned ecos==2.0.14 dependency needs an unavailable C++ compiler on this environment.

CheckList

• Did you add an unittest for your new example or defect fix?
• Did you add documents for your new example? (Not applicable: existing algorithm defect fix.)
• All CIs are green? (To be checked after submission.)