fix(agent): collision-safe thread keys

sdk/agent_session.py

@@ -10,7 +10,7 @@ from sdk.interface import MessageChunk, MessageResponse, PlatformError
 
 
 def build_thread_key(platform: str, user_id: str, chat_id: str) -> str:
-    return f"{platform}:{user_id}:{chat_id}"
+    return f"{len(platform)}:{platform}{len(user_id)}:{user_id}{len(chat_id)}:{chat_id}"
 
 
 @dataclass(frozen=True, slots=True)
@@ -75,6 +75,11 @@ class AgentSessionClient:
                             payload.get("details", "Agent error"),
                             code=payload.get("code", "AGENT_ERROR"),
                         )
+                    elif msg_type == "GRACEFUL_DISCONNECT":
+                        raise PlatformError(
+                            "Agent disconnected gracefully",
+                            code="GRACEFUL_DISCONNECT",
+                        )
                     else:
                         raise PlatformError(
                             f"Unexpected agent message: {payload}",