refactor: separate request auth channel from route kind

Extract request_auth_channel as a distinct routing dimension to distinguish
between api_key and bearer_like authentication methods. This improves routing
clarity by decoupling authentication mechanism from route classification.

apps/aether-gateway/src/tests/concurrency.rs

@@ -27,6 +27,7 @@ fn sample_decision() -> crate::control::GatewayControlDecision {
         route_class: Some("ai_public".to_string()),
         route_family: Some("openai".to_string()),
         route_kind: Some("chat".to_string()),
+        request_auth_channel: None,
         auth_endpoint_signature: None,
         execution_runtime_candidate: true,
         auth_context: None,