The Check protobuf has a `oneof config` field which generates separate
fields in the Elixir struct: :http, :tcp, :dns, :ssl. However,
check_type_config/2 was returning [config: {:http, ...}] which doesn't
match the struct definition, causing a KeyError crash in
build_and_push_check_jobs/1.
This bug was originally fixed in e0a74e2f but was accidentally
reintroduced in f703e61b when attempting to use "tagged tuple config".
Fix: Return field-specific keyword lists (e.g., [http: %HttpCheckConfig{}])
instead of [config: {:http, ...}].
Also added comprehensive test that creates all 4 check types and verifies
no KeyError crash occurs when sending check jobs to agent.
Reviewed-on: graham/towerops-web#141
