Fix canonicalization and matrix solution recovery bugs

[haozhu10015]

Summary

This PR fixes several uncovered bugs in expression canonicalization and solver solution recovery.

Changes

• Fix sum_squares canonicalization outside native objective form so constraints model ||x||², not ||x||.
• Fix quad_over_lin canonicalization to include the denominator in the SOC epigraph.
• Fix power(x, 2) so it remains elementwise instead of collapsing vector inputs through sum_squares.
• Add axis-aware canonicalization for sum_axis(..., 0) and sum_axis(..., 1).
• Fix sum_axis shape inference for axis = 1.
• Fix column-major element extraction for matrix-shaped affine expressions.
• Preserve original variable shapes through stuffing and solution unpacking, so matrix variables are returned as matrices instead of flattened vectors.

Tests

• Added tests/canon_tests.rs for canonicalization regressions; the previous sum_squares tests are merged here.
• Added tests/solver_tests.rs for matrix solution recovery.
• Added direct Expr::Sum(..., axis) shape unit coverage.