Cover service ClickHouse settings endpoints

[sdairs]

Summary

service_clickhouse_settings_list_get, service_clickhouse_settings_schema_get, service_clickhouse_setting_get, service_clickhouse_settings_update have no live coverage.

Scope

Extend tests/integration_test.rs with a ClickHouse Settings phase:

• service_clickhouse_settings_schema_get — fetch schema, assert non-empty.
• service_clickhouse_settings_list_get — fetch current settings, capture a known mutable setting's value.
• service_clickhouse_settings_update — change that setting to a different valid value.
• service_clickhouse_setting_get — confirm the change is visible.
• Restore the setting to its original value (idempotent post-state).

Where this lands

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

Notes

• Pick a setting that's safe to change on a running service and doesn't require a restart (e.g. max_concurrent_queries or similar). The schema endpoint should make valid options discoverable at test time.

Parent: #151