Cover postgres_instance_create_read_replica

[sdairs]

Summary

• Extends integration_postgres_test.rs with a Read Replica phase that exercises postgres_instance_create_read_replica end-to-end (create, poll-to-running, list+get verification including primary reference, explicit delete) before the primary Delete phase.
• Adds replica-id tracking to CleanupRegistry (register_postgres_replica / unregister_postgres_replica); the cleanup pass deletes registered replicas before registered primaries, since the live API refuses to delete a primary while a replica still references it.
• Replica is registered in the cleanup registry immediately on create so a panic between create and the in-body teardown cannot leak the replica and break the primary delete.

Cleanup ordering

Two complementary mechanisms keep replicas torn down before primaries:

1. CleanupRegistry — the cleanup pass iterates postgres_replica_ids to deletion before iterating postgres_ids. Registration happens immediately after the create call returns the id.
2. Explicit in-body teardown — the Read Replica phase deletes its replica and waits for 404 before the primary Delete phase runs. unregister_postgres_replica keeps the registry tidy on the happy path.

Both paths are documented in code comments (CleanupRegistry doc-comment and the Read Replica phase header in the test).

Test plan

• cargo build -p clickhouse-cloud-api
• cargo clippy -p clickhouse-cloud-api --tests -- -D warnings (only pre-existing errors in spec_coverage_test.rs and integration_test.rs remain; nothing in files touched by this PR)
• cargo test -p clickhouse-cloud-api (86 unit + 6 spec-coverage tests pass; integration_postgres test is #[ignore] per existing suite policy)
• Live cloud-integration run via PR CI to exercise the new phase end-to-end against production.

Closes #162

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