fix: validate gradient check points

[kholdrex]

Summary

Harden the gradient-check helper path so invalid base points are rejected before user-provided density or gradient closures run. This keeps finite-difference diagnostics consistent with the sampler constructors' finite, non-empty vector expectations.

Changes

• Add point validation for empty and non-finite vectors in finite_difference_gradient and gradient_check.
• Document the new error behavior in the gradient helper rustdoc.
• Extend regression coverage for empty, NaN, and infinite gradient-check points.

Test Plan

• Rust formatting check passed.
• Clippy passed across all targets and features with warnings denied.
• Full test suite passed across all features.
• Rustdoc generation passed with warnings denied.
• Benchmark compilation passed across all features.