fix(openai-ws): keep downstream writes off the relay cancellation context

coder/websocket arms a context.AfterFunc that hard-closes the connection
when a write context is canceled, and AfterFunc stop does not wait for a
callback that already started. An external cancellation (e.g. ingress
lease loss) landing inside the disarm window of an already-successful
downstream write could therefore kill the TCP connection before the
retry close frame (1013) was written, leaving the client with a bare
EOF. Mirror the read side: bound downstream writes with the write
timeout only, and rely on the explicit Close/CloseNow performed by every
relay exit path for teardown.

Fixes the flaky TestPassthroughLifecycle_LeaseLossSendsRetryClose.

backend/internal/service/openai_ws_v2/passthrough_relay.go

@@ -173,7 +173,13 @@ func Relay(
 		return writeUpstream(msgType, payload)
 	}
 	writeClient := func(msgType coderws.MessageType, payload []byte) error {
-		writeCtx, cancel := context.WithTimeout(relayCtx, writeTimeout)
+		// 下行写超时故意不挂在 relayCtx 上：coder/websocket 在已武装的 write
+		// ctx 被取消时会直接硬关连接（context.AfterFunc 的 stop 不等待执行中
+		// 的回调），外部取消若落在一次已成功写入的解除武装窗口内，会连同尚未
+		// 发出的 close 帧一起冲掉，客户端只能看到裸 EOF 而收不到关闭码。与读
+		// 侧 conn.Read(context.Background()) 同理，取消路径的连接回收由各退出
+		// 分支的显式 Close/CloseNow 兜底。
+		writeCtx, cancel := context.WithTimeout(context.Background(), writeTimeout)
 		defer cancel()
 		return clientConn.WriteFrame(writeCtx, msgType, payload)
 	}