test: cover service metadata round-trip and slot independence

[Jagadeeshftw]

Summary

Adds test-only coverage (in contracts/escrow/src/test.rs) proving the per-service storage slots are mutually independent:

• set_service_metadata then get_service_metadata round-trips the exact description + owner.
• A never-set service returns None from get_service_metadata.
• register_service does not set the disabled flag (is_service_disabled == false).
• set_service_disabled(true) preserves is_service_registered == true and the stored metadata.
• unregister_service clears only the ServiceRegistered slot — metadata and the disabled flag survive.
• A toggle-matrix test asserts is_service_registered / is_service_disabled move independently.

A module header note documents the slot model. No changes to lib.rs.

Closes #41

Test output

test result: ok. 29 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.21s

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

[mikewheeleer]

good — metadata round-trip and slot independence were untested. lgtm