[LinearSystem] ConstantSparsityPatternSystem: change key type to support bigger meshes

[fredroy]

Tried to increase the size of the beam in examples/Validation/cantilever_beam but came accross this error while running with 20x20x100 grid:
[ERROR] [ConstantSparsityPatternSystem(A)] Could not find index -446175222 (row 32074, col 32073) in the hash table
which smelled a lot like an overflow somewhere...

Culprit (thanks Claude) being the key type in the map of ConstantSparsityPatternSystem, which was a sofa::SignedIndex aka int32_t.
The flat index computation row + col * rows() overflows.

So the fix is to change this type to handle bigger values, with a int64_t

[with-all-tests]

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[fredroy]

[ci-build][with-all-tests]