Cover scaling_schedule_get / replace / upsert in service integration suite

[sdairs]

Summary

scaling_schedule_get, scaling_schedule_upsert, and scaling_schedule_replace (all Beta-tagged in the OpenAPI spec) have no live coverage. They were missed in the original #151 scope.

Scope

Extend tests/integration_test.rs with a Scaling Schedule phase, after the existing Scaling phase (so the service is in a known steady state):

• scaling_schedule_get — capture pre-test state (may legitimately be empty / unset).
• scaling_schedule_upsert — set a synthetic schedule (e.g. a single window with low min/max replicas that won't actually trigger during the test run).
• scaling_schedule_get — confirm the upsert is visible and round-trips.
• scaling_schedule_replace — replace with a different schedule. Verify via subsequent get.
• Restore the pre-test state (or delete the schedule if it didn't exist before). Idempotent post-state.

Where this lands

• crates/clickhouse-cloud-api/tests/integration_test.rs

Notes

• Pick schedule windows that won't actually scale the test service during the test window — these endpoints are Beta and we want shape coverage, not real scaling behaviour entangled with the rest of the suite.
• All three methods are Beta in crates/clickhouse-cloud-api/src/meta.rs — confirm the affordance still renders correctly if the CLI surfaces these.

Parent: #151