fix: dispatch PyLinearSolver::solve to the Python solve method

[oberbichler]

Problem

The solve override in the PyLinearSolver trampoline dispatched to the wrong Python method:

virtual bool solve(...) override {
    pybind11::gil_scoped_acquire acquire;
    PYBIND11_OVERRIDE_PURE(bool, T, factorize, ia, ja, a, b, x);  // names 'factorize', not 'solve'
}

When Problem::hm_inv_v calls solve(ia, ja, a, b, x) on a LinearSolver subclassed in Python, the trampoline invoked the Python factorize method (with 5 arguments) instead of the user's solve. A Python-defined linear solver therefore never had its solve executed.

Fix

Name solve in the macro, so the override dispatches to the Python solve.

Test

New tests/test_linear_solver_trampoline.py subclasses LinearSolver in Python (recording how it is called and applying a trivial x = b + 1), assigns it as problem.linear_solver, and calls problem.hm_inv_v. It asserts that solve is dispatched exactly once with the correct right-hand side, that factorize is only ever called with its 3-argument signature, and that the x written by solve flows back to the caller.

• Before the fix: solve is never called (factorize is invoked in its place); test fails.
• After the fix: passes; full suite 44 passed.