Consolidate the duplicated CRUD and IP-candidate test trios in api-db and health

[chet]

api-db and health carry duplication the helper was meant to remove: the same expected-entity CRUD suite written once per entity, an IP-candidate trio that differs only by input, and two tests that can't fail. Consolidate the trios into a shared helper or a table and drop the dead ones. Test-only; no behavior change.

Scope

api-db

• Merge the three expected_{machine,switch,power_shelf} CRUD suites -- test_lookup_by_mac, test_duplicate_fail_create, test_update_bmc_credentials, test_delete, identical modulo entity -- into a shared generic helper (each #[sqlx_test] still needs its own pool, so this is a parameterized helper, not a single table).
• Collapse the IPv4 candidate trio into one check_cases table (ip_allocator.rs); keep test_v4_single_ip_candidate (distinct).
• Drop the pure ORM round-trips in expected_rack/tests.rs (insert then read back unchanged).

health

• Delete test_should_monitor_key_consistency (sharding.rs) -- it asserts f(x) == f(x).
• Fix or delete test_sync_with_duplicate_addresses_in_input (route_servers.rs) -- it declares both Ok and Err acceptable, so it cannot fail; if there's a real uniqueness contract, assert it.

Acceptance

• The three CRUD suites run through one shared helper; coverage preserved.
• cargo test green for both crates; cargo clippy --all-features clean.

Size: S-M

Part of #2692.