fix: Improve error message clarity for test coverage failure

cover_agent/UnitTestValidator.py

@@ -530,7 +530,7 @@ def validate_test(self, generated_test: dict):
                         self.failed_test_runs.append(
                             {
                                 "code": fail_details["test"],
-                                "error_message": "Code coverage did not increase",
+                                "error_message": "Test did not increase code coverage",
                             }
                         )  # Append failure details to the list