fix(pylon): fence admission exclusions by request instance

Preserve the current engine-concurrency behavior while preventing stale requests from excluding replacement reservations. Update the new capacity regression fixture for request identities.

Refs: #1817

Author: barrygreengus

src/libraries/rust/stargate/crates/pylon-lib/src/queue_admission.rs

@@ -309,6 +309,9 @@ impl LiveRequestState {
                     let excluded_request = state
                         .requests
                         .get(&required.request_id)
+                        .filter(|record| {
+                            record.instance.as_ref() == Some(&required.request_instance)
+                        })
                         .and_then(|record| match &record.request {
                             LiveRequest::Queue(queue, _) => Some(queue),
                             LiveRequest::Observed(_) => None,
@@ -1004,6 +1007,28 @@ mod tests {
         }
     }
 
+    #[test]
+    fn admission_excludes_only_its_own_request_instance() {
+        let live = LiveRequestState::default();
+        live.update_model_throughput("model-a", 100.0);
+        let stale = required("reused-id", 0, 100);
+        let _first = live.track_request(&stale);
+        let current = required("reused-id", 0, 200);
+        let _replacement = live.track_request(&current);
+        let generation = ModelGeneration::new("model-a", 0);
+        let evaluate = |request| {
+            live.evaluate_generation(
+                &PylonQueueMismatchRetryConfig::default(),
+                request,
+                Some(&generation),
+                &headers_with_expected("0"),
+                Some(1),
+            )
+        };
+        assert_eq!(evaluate(&stale).actual_ms(), Some(2000));
+        assert_eq!(evaluate(&current).actual_ms(), Some(0));
+    }
+
     #[test]
     fn stale_guards_cannot_advance_or_remove_reused_request_ids() {
         for replacement_model in ["model-a", "model-b"] {

src/libraries/rust/stargate/crates/pylon-lib/src/stats/collector.rs

@@ -3724,6 +3724,7 @@ mod tests {
             2
         );
         let required = |request_id: &str| crate::request_observer::RequiredTunnelHeaders {
+            request_instance: Default::default(),
             request_id: request_id.to_string(),
             routing_key: None,
             model_id: "model-a".to_string(),