Cover postgres_instance_create_read_replica

[sdairs]

Summary

postgres_instance_create_read_replica has no live coverage.

Scope

Extend tests/integration_postgres_test.rs with a Read Replica phase, inserted after the primary is in running state:

• postgres_instance_create_read_replica against the primary service id. Register the replica id with the cleanup registry.
• Poll for the replica to reach running.
• Verify the replica appears in postgres_service_get_list and postgres_service_get returns the expected primary reference.
• Tear down the replica explicitly before the primary delete phase (and confirm via cleanup that no orphaned replicas remain if the test fails mid-way).

Where this lands

• crates/clickhouse-cloud-api/tests/integration_postgres_test.rs
• crates/clickhouse-cloud-api/tests/integration/support.rs — extend cleanup to handle replicas (likely just another postgres id, but the parent ref may matter for delete order).

Parent: #151