Add check_positive_decimal correctness function

[nicolad]

Introduce check_positive_decimal in crates/core/src/correctness.rs to enforce that rust_decimal::Decimal parameters are strictly positive. The helper replaces hand-rolled checks inside validate_instrument_common, tightening validation for fields such as margin_init. Comprehensive rstest cases cover positive, zero, and negative edge conditions.

Impact

Non-breaking: only invalid (≤ 0) decimal inputs now fail early with a clear error message, improving safety while leaving existing valid workflows unchanged.

Notes

• Added function‐level docs and #[inline(always)] hint.
• Extended unit-test suite in both correctness and model::instruments modules.

[cjdsellers]

Many thanks @nicolad !

[cjdsellers]

👌