fix: canonicalize canceled workbench final response

internal/cloud/workbench-projection-writer.test.ts

@@ -128,6 +128,7 @@ test("workbench projection writer seals canceled AgentRun turns with canonical c
     payload: {
       traceId: "trc_writer_canceled",
       status: "canceled",
+      finalResponse: { text: "command cancelled before backend start", status: "canceled", traceId: "trc_writer_canceled" },
       userMessage: "当前 AgentRun 请求已取消；traceId/runId/commandId 已保留，可重试上一条消息。",
       runnerTrace: {
         traceId: "trc_writer_canceled",

internal/cloud/workbench-turn-projection.ts

@@ -188,9 +188,13 @@ function terminalTurnEvidence({ result = null, traceTerminal = null } = {}) {
 }
 
 function terminalFinalResponse(status, result = null, traceTerminal = null) {
+  if (normalizeWorkbenchStatus(status) === "canceled") return canonicalCancelFinalResponse(result, traceTerminal);
   const direct = traceTerminal?.finalResponse ?? result?.finalResponse;
   if (direct) return direct;
-  if (normalizeWorkbenchStatus(status) !== "canceled") return null;
+  return null;
+}
+
+function canonicalCancelFinalResponse(result = null, traceTerminal = null) {
   return {
     text: CANCEL_FINAL_RESPONSE_TEXT,
     status: "canceled",