Make `RaoResult::isSecure` systematically rely on CNEC's margin

[bqth29]

Describe the current behavior

Some RaoResult::isSecure implementations (eg the one related to FastRAO) rely on the functional cost of the objective function which is right in margin optimization but wrong in costly optimization (since the cost is always positive).

Describe the expected behavior

All implementations should align with the strategy used by RaoResultImpl to rely on CNECs' margins instead of the functional cost.

Describe the motivation

No response

Extra Information

No response