fix: run the outcall cycles migration regardless of subnet activity

[mraszyk]

The migration added in #10844, which brings the HTTPOutcalls / ECDSAOutcalls entries of SubnetMetrics::consumed_cycles_by_use_case up to the legacy scalar fields, was invoked from observe_consumed_cycles_with_use_case. That only fires when the subnet observes a subnet-level use case, and those are only observed on HTTP outcalls, threshold signature outcalls, canister deletion and cycles lost to dropped messages. A subnet that does none of these never runs the migration and its use-case entries stay stale indefinitely.

This is not hypothetical: on mainnet subnet
2fq7c-slacv-26cgz-vzbx2-2jrcs-5edph-i5s2j-tck77-c3rlz-iobzx-mqe, consumed_cycles_ecdsa_outcalls is 244_900_000_000_000 while the ECDSAOutcalls use-case entry is 228_370_000_000_000, a gap of 16_530_000_000_000 cycles. Its consumed_cycles_by_use_case_as_counters map is empty, which (since that map is incremented on the very same code path) shows that no subnet-level use case has been observed there since the counters were introduced in #9922, i.e. well before the migration was deployed.

The migration now runs from the scheduler's finish_round, which is documented as running unconditionally after each round and is also reached by the heap-delta early return and by checkpoint_round_with_no_execution. Running it once per round instead of per observation is equivalent, because the call sites bump the scalar field and the matching use-case entry by the same amount: max(entry, scalar) + delta == max(entry + delta, scalar + delta). It is also idempotent, so the extra invocations are harmless.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to ab33d4a.

Security Overview

• 🔎 Scanned files: 4 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=f62f6f98-4c7b-4c88-b8a4-701d61950a6b&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/execution_environment/src/scheduler.rs     Backfill the HTTP/ECDSA outcall cycles from legacy scalar fields into use cases in finish_round()
Enhancement	► rs/execution_environment/src/scheduler/tests/metrics.rs     Add test for migrating outcalls cycles into use cases on idle subnet
Enhancement	► rs/replicated_state/src/metadata_state.rs     Make migrate_outcalls_cycles_to_use_cases public and remove inlined migration call from observe path; update comments
Enhancement	► rs/replicated_state/src/metadata_state/tests.rs     Add tests for migrating outcalls scalar fields into use cases and related scenarios