Update policy.xml (#85)

---------

Co-authored-by: Simon Kurtz <simonkurtz@microsoft.com>

Author: gkaleta

labs/model-routing/policy.xml

@@ -1,24 +1,25 @@
 <policies>
     <inbound>
         <base />
-        <set-backend-service backend-id="@{
-            // Determine what backend pool we use.
-            switch(context.Request.MatchedParameters["deployment-id"]) {
-                case "gpt-35-turbo-1106":
-                    return "{backend-id-1}";
 
-                case "gpt-35-turbo-0125":
-                    return "{backend-id-2}";
-
-                case "gpt-4o-2024-05-13":
-                    return "{backend-id-3}";
-
-                default:
-                    return "{backend-id-1}";
-            }
-        }" />
+        <!-- Use a <choose> block to evaluate deployment-id -->
+        <choose>
+            <when condition="@(context.Request.MatchedParameters["deployment-id"] == "gpt-35-turbo-1106")">
+                <set-backend-service backend-id="{backend-id-1}" />
+            </when>
+            <when condition="@(context.Request.MatchedParameters["deployment-id"] == "gpt-35-turbo-0125")">
+                <set-backend-service backend-id="{backend-id-2}" />
+            </when>
+            <when condition="@(context.Request.MatchedParameters["deployment-id"] == "gpt-4o-2024-05-13")">
+                <set-backend-service backend-id="{backend-id-3}" />
+            </when>
+            <otherwise>
+                <set-backend-service backend-id="{backend-id-1}" />
+            </otherwise>
+        </choose>
 
         <authentication-managed-identity resource="https://cognitiveservices.azure.com" output-token-variable-name="managed-id-access-token" ignore-error="false" />
+
         <set-header name="Authorization" exists-action="override">
             <value>@("Bearer " + (string)context.Variables["managed-id-access-token"])</value>
         </set-header>