Wire OpenTelemetry into ClassifiersExample

Without `braintrust.openTelemetryCreate()` the eval runs against the
no-op tracer and no spans reach Braintrust, so the experiment shows up
empty in the UI despite the REST experiment record being created. Every
other example under examples/ already makes this call right after
`Braintrust.get()` (see e.g. examples/experiment/.../ExperimentExample.java).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Qard

examples/classifiers/src/main/java/dev/braintrust/examples/ClassifiersExample.java

@@ -26,6 +26,7 @@
 public class ClassifiersExample {
     public static void main(String[] args) throws Exception {
         var braintrust = Braintrust.get();
+        braintrust.openTelemetryCreate();
 
         // 1. Single-label classifier.
         Classifier<String, String> intentClassifier =