Test remove_price_tiers flat-price fallback

[emirykl]

Summary

• verify removing price tiers clears the stored tier schedule
• verify unchanged usage switches from tiered billing back to the flat service price
• verify removing tiers from a service without tiers is idempotent

Verification

• cargo fmt --all -- --check
• cargo build --locked
• cargo test test_remove_price_tiers -- --nocapture (3 passed)
• Full cargo test --locked: 332 passed; the unrelated existing test_transfer_service_ownership_genuine_transfer_emits_event test fails because it expects two events but receives one

Fixes #210

[mikewheeleer]

neat work @emirykl — thanks for the care. merging 🌱