feat(bill-payments): add due-between range query

[jamilahmadzai]

Closes #1019

Bounty: GrantFox OSS campaign for issue #1019

Summary

• add get_bills_due_between(owner, start, end, cursor, limit) for unpaid bill range queries over the existing owner index
• return InvalidDueDate for inverted ranges while keeping [start, end] inclusive
• cover owner isolation, paid-bill filtering, boundary dates, empty windows, and cursor pagination

Validation

• /Users/jamilpc/.cargo/bin/rustfmt --edition 2021 --check bill_payments/src/lib.rs bill_payments/src/test.rs
• /Users/jamilpc/.cargo/bin/cargo test -p bill_payments test_get_bills_due_between -- --nocapture
• /Users/jamilpc/.cargo/bin/cargo clippy -p bill_payments --all-targets (passes with existing warnings in test files)
• git diff --check

Full package note: /Users/jamilpc/.cargo/bin/cargo test -p bill_payments -- --nocapture currently has one pre-existing failure in events_schema_test::reserved_bill_event_variants_serialize_as_enum_not_symbol; the new due-between tests pass in that run.

[jamilahmadzai]

CI note after opening this PR:

• Touched package validation passed locally: rustfmt --edition 2021 --check bill_payments/src/lib.rs bill_payments/src/test.rs, cargo test -p bill_payments test_get_bills_due_between -- --nocapture, cargo clippy -p bill_payments --all-targets, and git diff --check.
• The full bill_payments test run was also attempted. It reaches this PR's new due-between tests, but the existing events_schema_test::reserved_bill_event_variants_serialize_as_enum_not_symbol assertion still fails outside the touched code.
• Final CI readback: Security Checks, Gas Benchmarks, and Storage Key Naming Convention Validation pass. Validate and Build, Build and Test, and Lint (unwrap/expect ban) fail while compiling the existing reporting crate (ToI128Checked not imported, IntoVal/FromVal derive macro issues, incompatible if branch types, and ? in calculate_savings_score returning u32), not in bill_payments.