Cleanup IManifoldManager and shape methods

[metalgearsloth]

Moved radius setters into a standard area to share code.
Same with the IPhysShape methods so we can duplicate less.
IManifoldManager got moved to sharedphysicssystem so content can use it if it wants.

1. ChildCount is liable to get removed if I ever finish v3 port and I hate seeing it on the objects. Also makes it easier to do future updates having the 1 spot and not across a bunch of areas.
2. Radius validation was inconsistent.
3. ShapeType was a C++ism that wasn't needed.
4. IManifoldManager can be useful.

[rebaserHEAD]

Shipfork enjoyers watching with great interest.

[DrSmugleaf]

  Failed TestSpawnTraversal [237 ms]
  Error Message:
   Robust.Shared.IoC.Exceptions.UnregisteredDependencyException : Robust.Shared.Physics.Systems.FixturesChangeSystem requested unregistered type with a dependency field: Robust.Shared.Physics.Systems.SharedPhysicsSystem

From https://github.com/Space-Wizards-Federation/RobustToolbox/actions/runs/27812117239/job/82304667588?pr=43