fix: Guard against None tracker in Judge.evaluate()

Raise a clear RuntimeError if create_tracker returns None rather
than letting it crash with AttributeError on track_metrics_of_async.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jsonbailey

packages/sdk/server-ai/src/ldai/judge/__init__.py

@@ -71,6 +71,11 @@ async def evaluate(
             judge_result.sampled = True
 
             tracker = self._ai_config.create_tracker()
+            if tracker is None:
+                raise RuntimeError(
+                    "AIConfig.create_tracker returned None. "
+                    "Ensure the config was obtained from the client rather than constructed directly."
+                )
             messages = self._construct_evaluation_messages(input_text, output_text)
             assert self._evaluation_response_structure is not None