[Framwork] Fix 'unused' warnings (and some others)

[fredroy]

courtesy of Qwen next + opencode.
He also did the tests (where nobody gives a damn 😶‍🌫️)

• DrawColoredMesh: SOFA_UNUSED() the parameters of the empty doDraw specializations for Edge/Triangle/Quad/Prism/Pyramid.
• IdentityMatrix: SOFA_UNUSED() the identity operand in the operators that do not consume it.
• MechanicalPropagateOnlyVelocityVisitor: SOFA_UNUSED(time).
• Visitor_test: SOFA_UNUSED() the parameters of the override stubs.
• TopologySubsetIndices_test: mark init/getTopologyType/getNbPoints as override (they override base-class virtuals).
• Proximity_test: static_cast() the [[nodiscard]] call.
• Triangle_test: drop two unused 'using Scalar = ...' aliases.

[with-all-tests]

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• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).