Add print_test_errors to display test failures in UI

- Call Test.print_test_errors(ts) when tests fail during grading
- This ensures test failure details are visible in the UI output
- Test errors show with stack traces and expressions that failed
- Helps users debug why their solution received a score of 0

Author: Keno

src/tools/grade_problem.jl

@@ -92,6 +92,11 @@ function ClaudeMCPTools.execute(tool::GradeProblemTool, params::Dict)
 
         has_test_failures = has_failures(ts)
 
+        # Print test errors to stdout so they appear in the UI
+        if has_test_failures
+            Test.print_test_errors(ts)
+        end
+        
         # Debug: Print the result type
         @debug "Grade function returned: $(typeof(result))"