addressed copilot comments

Author: bachuv

client/src/main/java/com/microsoft/durabletask/DurableTaskGrpcClient.java

@@ -345,6 +345,7 @@ public PurgeResult purgeInstances(PurgeInstanceCriteria purgeInstanceCriteria) t
 
     @Override
     public void rewindInstance(String instanceId, @Nullable String reason) {
+        Helpers.throwIfArgumentNull(instanceId, "instanceId");
         RewindInstanceRequest.Builder rewindRequestBuilder = RewindInstanceRequest.newBuilder();
         rewindRequestBuilder.setInstanceId(instanceId);
         if (reason != null) {

endtoendtests/src/test/java/com/functions/EndToEndTests.java

@@ -32,7 +32,16 @@ public static void setup() {
         RestAssured.baseURI = "http://localhost";
         // Use port 8080 for Docker, 7071 for local func start
         String port = System.getenv("FUNCTIONS_PORT");
-        RestAssured.port = port != null ? Integer.parseInt(port) : 8080;
+        if (port != null) {
+            try {
+                RestAssured.port = Integer.parseInt(port);
+            } catch (NumberFormatException e) {
+                throw new IllegalArgumentException(
+                    "FUNCTIONS_PORT environment variable must be a valid integer, but was: '" + port + "'", e);
+            }
+        } else {
+            RestAssured.port = 8080;
+        }
     }
 
     @Order(1)