Cover instance_password_update + instance_scaling_update

[sdairs]

Summary

instance_password_update and instance_scaling_update (the vertical-only scaling endpoint, distinct from instance_replica_scaling_update) have no live coverage.

Scope

Extend tests/integration_test.rs with two new phases:

Password update

• instance_password_update rotates the service password. Assert the response surfaces a new password.
• The service-level password isn't used elsewhere in the test (the query flow uses an openapi key) — a successful response is the pass condition.

Vertical scaling

• Add a phase to the existing Scaling section using instance_scaling_update instead of instance_replica_scaling_update. Round-trip memory the same way (e.g. 8 → 16 → 8 GB), polling for the change to land.
• Confirm post-condition equals pre-condition so the rest of the scaling phase observes the expected base state.

Where this lands

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

Parent: #151