Add typed event payload modules for the streaming service.

surfsense_backend/app/services/streaming/events/error.py

@@ -0,0 +1,23 @@
+"""Single terminal error path the orchestrator must route through."""
+
+from __future__ import annotations
+
+from typing import Any
+
+from ..emitter import Emitter, attach_emitted_by
+from ..envelope import format_sse
+
+
+def format_error(
+    error_text: str,
+    *,
+    error_code: str | None = None,
+    extra: dict[str, Any] | None = None,
+    emitter: Emitter | None = None,
+) -> str:
+    payload: dict[str, Any] = {"type": "error", "errorText": error_text}
+    if error_code:
+        payload["errorCode"] = error_code
+    if extra:
+        payload.update(extra)
+    return format_sse(attach_emitted_by(payload, emitter))