fix: return InvalidAmount on unpaid total overflow

[brentthomas248]

Closes #1018

Summary:

• replace saturating unpaid-total cache updates with checked arithmetic that returns InvalidAmount on overflow or underflow
• propagate checked unpaid-total failures through create_bill, pay_bill, batch_pay_bills, cancel_bill, and schedule execution
• add regression coverage for overflow/underflow state preservation and update stale large-amount/pagination baseline tests

Tests:

• PATH="$HOME/.cargo/bin:$PATH" cargo test -p bill_payments
• PATH="$HOME/.cargo/bin:$PATH" cargo clippy -p bill_payments --all-targets -- -D warnings

CI note:

• Public workspace CI is currently failing in unrelated reporting crate checks before this bill_payments patch is implicated. The focused bill_payments test/clippy commands above pass locally.