Table-ify ssh-console cert + escape-sequence tests; cover BmcVendor round-trip

[chet]

ssh-console has two hand-rolled test clusters plus an untested string contract. ssh_cert_parsing.rs holds ~14 single-assertion tests over key_id_contains_role / get_user_from_key_id (each (key_id, role/fmt) -> bool / Option<&str>), and bmc/vendor.rs has one fat test_filter_escape_sequence (~15 inline assert_eq! blocks with no per-case labels).

Collapse the cert tests into value_scenarios! tables and the escape-sequence test into a scenarios! table over (input, prev_pending) -> (output, pending_out) so each case is named. Then ADD the missing BmcVendor round-trip coverage: from_config_string / config_string (6 vendors, string<->enum) and its custom serde -- currently zero tests on a contract used in config (de)serialization.

Scope

• crates/ssh-console/src/ssh_cert_parsing.rs -- collapse the 14 predicate tests.
• crates/ssh-console/src/bmc/vendor.rs -- table-ify test_filter_escape_sequence; add BmcVendor round-trip + serde tables.

Acceptance

• Cert/escape coverage preserved; new BmcVendor round-trip table covers all 6 variants + an invalid string.
• cargo test -p ssh-console green; cargo clippy --all-features + cargo make format-nightly clean.

Size: L

Part of #2692.