Test for duplicate transport versions (elastic#130494)

We used to have an assertion during transport version loading that
duplicate ids were not found, but it appears to have been lost in
refactorings. This commit adds a test to ensure duplicate ids do not
occur.

relates elastic#130486

Author: rjernst

server/src/test/java/org/elasticsearch/TransportVersionTests.java

@@ -15,6 +15,7 @@
 import java.lang.reflect.Modifier;
 import java.util.Collections;
 import java.util.Map;
+import java.util.List;
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.regex.Matcher;
@@ -256,4 +257,17 @@ public void testDenseTransportVersions() {
             fail(msg.toString());
         }
     }
+
+
+    public void testDuplicateConstants() {
+        List<TransportVersion> tvs = TransportVersions.getAllVersions().stream().sorted().toList();
+        TransportVersion previous = tvs.get(0);
+        for (int i = 1; i < tvs.size(); i++) {
+            TransportVersion next = tvs.get(i);
+            if (next.id() == previous.id()) {
+                throw new AssertionError("Duplicate transport version id: " + next.id());
+            }
+            previous = next;
+        }
+    }
 }