fix(test): use deterministic echo prompt for session scenarios (#5)

Align with Go/TS: 'Reply with exactly <marker>' instead of asking the
model for a free-form introduction with a trailing marker. The old
Chinese prompt gave the model room to refuse echoing the random string.

Author: moonyue-w

tests/support/scenarios/forward.py

@@ -49,7 +49,7 @@ def session(client: Forward, context: Run) -> None:
     session_id = context.track("session", session.id, lambda: finish_session(client, context, session.id))
 
     expected = marker()
-    prompt = "请用一句话介绍你能提供什么帮助，并在末尾原样附上：" + expected
+    prompt = "Reply with exactly " + expected
     context.output("user", prompt)
     sent = client.sessions.events.send(
         session_id,

tests/support/scenarios/managed.py

@@ -41,7 +41,7 @@ def session(client: Managed, context: Run) -> None:
     session_id = context.track("session", session.id, lambda: finish_session(client, context, session.id))
 
     expected = marker()
-    prompt = "请用一句话介绍你能提供什么帮助，并在末尾原样附上：" + expected
+    prompt = "Reply with exactly " + expected
     context.output("user", prompt)
     sent = client.sessions.events.send(
         session_id,