feat: add persisted batch text testing

backend/services/test_runs/errors.py

@@ -0,0 +1,24 @@
+"""Structured failures shared by the text runner and batch orchestrator."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass
+class TestExecutionError(RuntimeError):
+    code: str
+    message: str
+    stage: str
+    retryable: bool = False
+
+    def __post_init__(self) -> None:
+        RuntimeError.__init__(self, self.message)
+
+    def as_dict(self) -> dict[str, object]:
+        return {
+            "code": self.code,
+            "message": self.message,
+            "stage": self.stage,
+            "retryable": self.retryable,
+        }