fix: use BigInt::from(-1) instead of invalid -1 % field

[GarmashAlex]

• Replace non-compilable BigInt construction BigInt::from(-1 % field) in Substitution::substitution_into_constraint with BigInt::from(-1).
• Aligns with existing modular handling across the codebase, where reduction happens in modular_arithmetic operations.
• Grepped the repository; no other analogous invalid usages found.