Fix NaN step size after a failed step with complex y

[etiferrier]

When an implicit step's root find fails, the solver reports y_error = inf, which is inf + 0j for complex y. PIDController then divides it by the real tolerance scale atol + rtol * |y|. JAX promotes the scale to complex, and complex division computes inf * 0 = nan for the imaginary part, so the scaled error is inf + nanj. Every lineax norm of that is NaN (jnp.abs(inf + nanj) is NaN in JAX), so the next step size is NaN and the solve loops until max_steps. With real y the same path gives inf: the step is rejected and dt shrinks, as intended.

Divide the real and imaginary parts separately instead. This only changes complex y, and gives the same values as before whenever the error is finite. The fix is in the controller rather than where y_error = inf is set, because the error must keep the dtype of y, and any complex inf divided by a promoted real gives a NaN. The controller also covers every source of inf: the Runge-Kutta and implicit Euler failure paths and the NaN-to-inf mapping in _integrate.py.

This affects every implicit solver (ImplicitEuler, Kvaerno*, KenCarp*) with complex y: any step whose root find fails ends the solve. In v0.7.2, where VeryChord gives up after two iterations (before #754), this already happens with the default dt0, e.g. Kvaerno5 on -1j * (sz + cos(t) * sx) @ y.

Fixes #774.