Add ResolveConstraints logic to JointSliders (=> ModelVisualizer)

[RussTedrake]

Updates JointSliders to have discrete state (previously it was stateless). Additionally, if the new ResolveConstraints logic is active, we publish the (rounded) resolved constraints back to Meshcat, so that the other sliders are updated.

Since ModelVisualizer uses JointSliders, now ModelVisualizer also satisfies the multibody constraints.

Resolves https://github.com/RobotLocomotion/drake/issues/18917.

Deprecates JointSliders::SetPositions(q), which has been replaced with JointSliders::SetPositions(context, q).

https://github.com/user-attachments/assets/3abb096e-d4ef-48fb-8976-b5729beb7f9e

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This change is