test: cover settle event payload and drain-to-zero invariant

[Jagadeeshftw]

Summary

Adds test-only coverage (in contracts/escrow/src/test.rs) for the settlement path invariants:

• After settle, the per-(agent, service) usage counter drains to 0 and get_last_settlement returns Some(ts) matching a clock set via env.ledger().with_mut.
• The billed value returned by settle equals compute_billing for the same pre-settle state.
• settle emits a settled event with payload (agent, service_id, requests, billed).
• record_usage emits a usage event with payload (agent, service_id, requests, total).
• Settling a zero-usage pair returns 0, still stamps LastSettlement, and still emits a settled event.

A module header comment documents the covered invariants. No changes to lib.rs.

Closes #40

Test output

test result: ok. 28 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.22s

Security notes: Test-only change; no contract logic, storage layout, or auth surface is modified.

[mikewheeleer]

nice, the settle drain-to-zero invariant deserves a test. lgtm