MILP trail search methods for one trail within a weight range

[p-huynh]

Currently, the MILP xor differential/linear models only allow the user to find one optimal trail or one trail with an exact weight. This PR adds two newfind_one_xor_*_trail_with_weight_at_most methods:

• They take an upper bound (and optionally a lower bound), and return a trail whose weight falls within the range [0..upper_bound] (or [lower_boud..upper_bound]), mirroring the existing find_all_*_trails_with_weight_at_most methods.
• The old existing fixed weight methods now just call these with the lower and upper bound set to the same value.

Additional cleanup:

• Simplified weight_constraints, and removed an old unused trick where a negative weight quietly meant "at most". It only existed in the MILP model and nothing used it.
• Rewrote find_all_..._with_weight_at_most to add a single range constraint and collect the trails in one pass, instead of looping over every weight value. It returns the same trails, with fewer solver calls.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
97.0% Coverage on New Code
0.0% Duplication on New Code

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