fix: prevent negative merchant amounts in fee split

[Adecom16]

Fixes #355

Prevents calculate_split from producing a negative merchant_amount when independently rounded-up fees exceed the payment amount.

Changes

• Added validation for invalid fee splits.
• Prevented fee calculations from extracting more than the payment amount.
• Added regression tests covering rounding and over-extraction cases.
• Updated affected settlement/payment tests.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test

All relevant checks pass.

Impact

Valid fee calculations remain unchanged. Invalid fee configurations now fail safely instead of storing or processing a negative merchant payout.