[Diffusion] Explicit MechanicalObject link

[GaetanDesrues]

Adding an explicit link to the MechanicalObject instead of relying on tags

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[fredroy]

I see GaetanDesrues:v25.12 -> sofa-framework:v25.12
I think this PR should be merged into master, and might be backported to v25.12 later (or not)

[bakpaul]

Hello, all ForceFields are SingleStateAccessor, which are member called mstate and already are a link to the mechanical state. Can you check if this fits your need ?