fix(routing): preserve fixed order for streaming chat

apps/aether-gateway/src/ai_serving/planner/standard/openai/chat/plans/stream.rs

@@ -1,4 +1,4 @@
-use aether_routing_core::RoutingExecutionPolicy;
+use aether_routing_core::{RoutingExecutionPolicy, RoutingSchedulingMode};
 use async_trait::async_trait;
 use std::collections::VecDeque;
 use tracing::warn;
@@ -207,7 +207,12 @@ impl LocalOpenAiChatStreamAttemptSource<'_> {
     async fn next_raw_attempt_with_target_select(
         &mut self,
     ) -> Result<Option<LocalOpenAiChatCandidateAttempt>, GatewayError> {
-        let select_window = openai_chat_stream_target_select_window();
+        let select_window = openai_chat_stream_target_select_window_for_mode(
+            self.input
+                .routing_policy
+                .as_ref()
+                .map(|policy| policy.scheduling_mode),
+        );
         if select_window <= 1 {
             return self.next_raw_attempt_linear().await;
         }
@@ -365,6 +370,15 @@ fn openai_chat_stream_target_select_window() -> usize {
         .clamp(1, MAX_OPENAI_CHAT_STREAM_TARGET_SELECT_WINDOW)
 }
 
+fn openai_chat_stream_target_select_window_for_mode(
+    scheduling_mode: Option<RoutingSchedulingMode>,
+) -> usize {
+    if scheduling_mode == Some(RoutingSchedulingMode::FixedOrder) {
+        return 1;
+    }
+    openai_chat_stream_target_select_window()
+}
+
 #[derive(Clone, Copy)]
 struct TargetSelectCandidateIdentity<'a> {
     provider_id: &'a str,
@@ -574,4 +588,14 @@ mod tests {
 
         assert_eq!(select_target_index(19, &choices), 1);
     }
+
+    #[test]
+    fn fixed_order_disables_stream_target_selection() {
+        assert_eq!(
+            openai_chat_stream_target_select_window_for_mode(Some(
+                RoutingSchedulingMode::FixedOrder,
+            )),
+            1
+        );
+    }
 }