do not show unknown for custom evaluators (Azure#41335)

Author: slister1001

sdk/evaluation/azure-ai-evaluation/azure/ai/evaluation/_evaluate/_evaluate.py

@@ -1133,8 +1133,8 @@ def _map_names_to_builtins(
     ) -> Dict[str, str]:
     """
     Construct a mapping from user-supplied evaluator names to which known, built-in
-    evaluator or grader they refer to. Custom or otherwise unknown evaluators are
-    mapped to the "unknown" value.
+    evaluator or grader they refer to. Custom evaluators are excluded from the mapping
+    as we only want to track built-in evaluators and graders.
 
     :param evaluators: The dictionary of evaluators.
     :type evaluators: Dict[str, Callable]
@@ -1156,8 +1156,8 @@ def _map_names_to_builtins(
                 found_eval = True
                 break
         if not found_eval:
-            # If not found, map to "unknown"
-            name_map[name] = "unknown"
+            # Skip custom evaluators - we only want to track built-in evaluators
+            pass
 
     for  name, grader in graders.items():
         name_map[name] = grader.id