fix: lower the consumed cycles gauge on a full refund

[mraszyk]

SystemState::observe_consumed_cycles_with_use_case() maintains two kinds of metrics: the gauges (consumed_cycles and consumed_cycles_by_use_cases), which are raised by the prepayment and lowered by the refund; and the monotonic counters (consumed_cycles_by_use_cases_as_counters), which for the refundable use cases (Instructions and RequestAndResponseTransmission) are only bumped at refund time, by prepayment - refund.

The early return conflated the two: for a refund equal to its prepayment it skipped the update altogether, so the gauges kept the prepayment even though nothing was consumed in the end. The inflated value also outlives the canister, as CanisterManager::delete_canister() folds consumed_cycles_by_use_cases into the subnet level metrics.

Skip only if there is nothing to record at all, i.e. if both the prepayment and the refund are zero. Non-refundable use cases are unaffected, as a zero prepayment implies a zero refund for them, and NominalCycles saturates on subtraction, so a refund against a freshly created entry cannot underflow.

Tests: the new full_refund_resets_consumed_cycles refunds a prepayment in full and checks that the balance and both gauges are back to where they started and that the counter did not move. It covers both refundable use cases under both cost schedules. The gauges are kept in nominal cycles, which the cost schedule does not affect, so a full refund under Free hits the same skipped update; it is in fact the more interesting case, as Free waives the real amount and hence leaves the metrics as the only observable of the prepayment. The assertion on the balance after the prepayment (which drops by the real amount under Normal and stays put under Free) pins down that difference. Each of the four combinations fails without the fix.

[Copilot]

Pull request overview

Fixes consumed-cycle gauges remaining inflated after a full refund.

Changes:

• Updates metric handling to process equal prepayment/refund amounts.
• Adds coverage for both refundable use cases and cost schedules.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rs/replicated_state/src/canister_state/system_state.rs	Corrects the metric-update early return.
rs/replicated_state/src/canister_state/tests.rs	Tests full-refund balance, gauges, and counters.

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[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 594a004.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=286c57f5-00d5-4696-a15a-557550f30dc3&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Bug Fix	► rs/replicated_state/src/canister_state/system_state.rs       Skip only if there is nothing to record at all. Note that a refund equal to its prepayment still has to lower the gauge by the refunded amount, even though it contributes nothing to the monotonic counter.
Enhancement	► rs/replicated_state/src/canister_state/tests.rs       Add test: full refund resets consumed cycles ► rs/replicated_state/src/canister_state/tests.rs       Extend imports to include CyclesUseCaseRefundableKind and related types ► rs/replicated_state/src/canister_state/tests.rs       Implement test logic for full refund behavior across CanisterCyclesCostSchedule variants