fix: merge InferencePool route configs during HTTPRoute merge (#3964)

pkg/ingress/kube/gateway/istio/testdata/http.yaml

@@ -421,3 +421,80 @@ spec:
     name: vllm-llama3-8b-instruct-epp
     port:
       number: 9002
+---
+# Test case for multiple HTTPRoutes with InferencePools on same gateway.
+# This verifies that InferencePool configs in Config.Extra are preserved when
+# the routes are merged into a single VirtualService.
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: HTTPRoute
+metadata:
+  name: multi-route-infpool-1
+  namespace: default
+spec:
+  parentRefs:
+    - name: gateway
+      namespace: higress-system
+  hostnames: ["multi-infpool.domain.example"]
+  rules:
+    - matches:
+        - path:
+            type: PathPrefix
+            value: /model1
+      backendRefs:
+        - name: infpool-model1
+          group: inference.networking.k8s.io
+          kind: InferencePool
+          port: 80
+---
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: HTTPRoute
+metadata:
+  name: multi-route-infpool-2
+  namespace: default
+spec:
+  parentRefs:
+    - name: gateway
+      namespace: higress-system
+  hostnames: ["multi-infpool.domain.example"]
+  rules:
+    - matches:
+        - path:
+            type: PathPrefix
+            value: /model2
+      backendRefs:
+        - name: infpool-model2
+          group: inference.networking.k8s.io
+          kind: InferencePool
+          port: 80
+---
+apiVersion: inference.networking.k8s.io/v1
+kind: InferencePool
+metadata:
+  name: infpool-model1
+  namespace: default
+spec:
+  targetPorts:
+    - number: 8000
+  selector:
+    matchLabels:
+      app: model1-server
+  endpointPickerRef:
+    name: model1-epp
+    port:
+      number: 9002
+---
+apiVersion: inference.networking.k8s.io/v1
+kind: InferencePool
+metadata:
+  name: infpool-model2
+  namespace: default
+spec:
+  targetPorts:
+    - number: 8000
+  selector:
+    matchLabels:
+      app: model2-server
+  endpointPickerRef:
+    name: model2-epp
+    port:
+      number: 9002