Treat a cancelled SSE stream as aborted

Why: the race detector can deliver message_end when the context is already cancelled, because select picks either ready case.
Evidence: GOWORK=off go test -count=50 -race ./runtime/llm/ passed, including TestParseSSECancel.
Impact: a cancelled parse ends with the aborted event.
Refs: D1
Co-authored-by: Cursor <cursoragent@cursor.com>

runtime/llm/sse.go

@@ -151,6 +151,12 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 		emitError(out, content.String(), true, err.Error())
 		return
 	}
+	// 取消必须压过正常结束。select 在两个分支都就绪时随机挑选，
+	// 竞态下会把 message_end 发给已经取消的调用方。
+	if ctx.Err() != nil {
+		emitAbort(out, content.String(), ctx.Err())
+		return
+	}
 	msg := Message{Role: RoleAssistant, Content: content.String(), StopReason: mapFinish(finished, len(order) > 0)}
 	for _, idx := range order {
 		msg.ToolCalls = append(msg.ToolCalls, *calls[idx])