Fix Rust codegen matrix return types

[tandede]

Problem

Rust codegen formats every matrix returned directly from a generated function as nalgebra::SVector<scalar, rows * cols>. This loses the original two-dimensional shape. For example, a function returning Matrix23 is currently generated with an SVector<f64, 6> signature and constructor.

The multi-output path already handles this correctly through format_typename, which selects SVector only when either matrix dimension is one and otherwise selects SMatrix.

Approach

Reuse the same shape-aware formatter in both places used by the single-return path:

• the generated function return type;
• the constructor used for the returned value.

This keeps the single-return and multi-output paths consistent without duplicating their vector-versus-matrix condition. Scalar returns and vector-shaped matrices retain their existing behavior.

Regression coverage

The Rust codegen test now includes a non-square 2×3 matrix return. A non-square shape verifies both dimensions explicitly instead of checking only the flattened element count.

Before this change, the generated code used:

nalgebra::SVector<f64, 6>

It now uses:

nalgebra::SMatrix<f64, 2, 3>

The generated crate still includes the existing 3×1 return case, which remains an SVector<f64, 3>.

Validation

• Rust codegen test with the SymEngine backend
• Rust codegen test with the SymPy backend
• Cargo build of both generated test-data crates
• Ruff 0.9.10 lint and formatting checks
• git diff --check

Fixes #439