fix: harden sampler input validation

[kholdrex]

Summary

Harden sampler input validation so invalid numerical parameters are rejected before sampling starts. Metropolis-Hastings, Hamiltonian Monte Carlo, and Gibbs samplers now fail fast for zero-dimensional initial states and non-finite initial values, and MH/HMC parameter setters reject non-finite positive-only inputs.

Changes

• Add shared validation for non-empty, finite sampler initial states.
• Require MH proposal standard deviations, HMC step sizes, and HMC mass matrix entries to be positive and finite.
• Add regression coverage for NaN, infinity, non-positive parameters, zero-dimensional states, and non-finite initial states.

Test Plan

• Rust formatting check passed.
• Clippy passed with warnings denied across all targets and features.
• Full all-features test suite passed.
• Rust documentation built with warnings denied.
• Benchmark targets compiled successfully.