Add typed event payload modules for the streaming service.

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

@@ -0,0 +1,29 @@
+"""High-level message and step lifecycle events.
+
+Wire verbs are fixed by the AI SDK protocol (``start`` / ``finish`` for
+the whole message, ``start-step`` / ``finish-step`` for each step).
+Python helpers always read ``format_<entity>_<verb>`` so pairs are
+visible at the call site.
+"""
+
+from __future__ import annotations
+
+from ..emitter import Emitter, attach_emitted_by
+from ..envelope import format_sse
+
+
+def format_message_start(message_id: str, *, emitter: Emitter | None = None) -> str:
+    payload = {"type": "start", "messageId": message_id}
+    return format_sse(attach_emitted_by(payload, emitter))
+
+
+def format_message_finish(*, emitter: Emitter | None = None) -> str:
+    return format_sse(attach_emitted_by({"type": "finish"}, emitter))
+
+
+def format_step_start(*, emitter: Emitter | None = None) -> str:
+    return format_sse(attach_emitted_by({"type": "start-step"}, emitter))
+
+
+def format_step_finish(*, emitter: Emitter | None = None) -> str:
+    return format_sse(attach_emitted_by({"type": "finish-step"}, emitter))