Function pre_read_default_agent_kind 

pub fn pre_read_default_agent_kind(val: &Value) -> AgentKind

Pull default_agent_kind out of the raw BP JSON top level. Falls back to the schema’s Default impl (Operator) if the key is missing or its type does not match. This is the first stage of resolving the default kind used inside expand_file_refs when a $agent_md has no sibling kind override.