Refactor step-function dispatch (_build_step_func)

[jc-macdonald]

Refactor the _build_step_func dispatch in core_solver.py (around L1850) to reduce cyclomatic complexity.

Problem

The current dispatch logic triggers C901 (too complex) and is suppressed with a noqa comment. As solver methods grow, this will become increasingly unmaintainable.

Proposed approach

Replace the if/elif chain with a _PLAN_TO_STEP_FUNC mapping dict as Tim suggested:

_PLAN_TO_STEP_FUNC: dict[MethodName, Callable] = { ... }

if (step_func := _PLAN_TO_STEP_FUNC.get(plan.method)) is not None:
    if plan.jacobian is None:
        raise RuntimeError(_INTERNAL_ERROR_OP_AXIS_MSG)
    return step_func(rhs_func, step=step, jacobian=plan.jacobian)

Related

• Add new solver stepping methods #27 (new solver methods add more branches to dispatch)

Ref: PR #18 review by @TimothyWillard (L1850, L1918)