BUG: `tune=False` is not respected for sequential sampling

Describe the issue:

Single process MCMC sets step.tune = True any time the number of tuning steps is nonzero (due to this line).

Discovered this when trying to debug a model that was throwing exceptions during tuning, regardless of whether or not I enabled or disabled tuning at the step method constructor.

This is not the same behavior as multiprocess MCMC (which does no such adjustment). And further, it has different effects depending on the step method. For DEMetropolis(Z), setting tune=True actually doesn't cause tuning to happen, because the tuning target is set to None (rather than lambda or scaling). But in Slice, and I think most others, tuning does get turned back on.

Due to these discrepancies I suppose this is a bug.

Reproduceable code example:

<error noticed in the debugger>

Error message:

No response

PyMC version information:

%watermark -p pymc,pytensor
pymc    : 5.0.2
pytensor: 2.9.1

Context for the issue:

No response

Start in pymc/sampling/mcmc.py around lines 812-814 and trace where single-process sampling sets step.tune. Compare that behavior with multiprocess MCMC and the step-method cases described for DEMetropolis(Z) and Slice. Done means sequential sampling respects the constructor's tune=False setting consistently.