Adopt carbide-test-support across the agent crate's parser tests

[chet]

The agent crate doesn't use the table helper at all, yet it's full of per-case parser/predicate clusters -- the largest single adoption target in Wave 2. This adopts carbide-test-support across its parser tests and closes a few coverage gaps the per-case style left open.

Scope

Adopt the helper (collapse per-case clusters):

• health/bgp.rs -- the 8 verify_bgp_summary tests (each rebuilds the same scaffold) -> one check_cases table keyed on a named struct ({ json, host_routes, route_servers, expected_alerts }).
• nvue.rs -- split_prefixes_by_family (7 near-identical) -> value_scenarios!.
• command_line.rs -- AgentPlatformType / HbnConfigMode from_str + the for value in [...] loop -> tables.
• health.rs -- parse_status / parse_disk_utilization / parse_mlxprivhost -> tables.

Add coverage (table-shaped, currently thin/absent):

• health.rs -- SctlState from_str + Display (8 variants, ~3 tested; Unknown fallthrough untested).
• command_line.rs -- HbnConfigMode::from_str (zero tests today).
• containerd/image.rs -- container_image_name_to_component regex (no direct unit tests).

Add carbide-test-support to [dev-dependencies].

Acceptance

• Adopted clusters preserve coverage; the added tables genuinely raise it.
• cargo test -p <agent pkg> green; cargo clippy --all-features + cargo make format-nightly clean.

Size: M-L

Part of #2692.