fix(chat): avoid implicit stream reuse

tests/routes/chatSession.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from "bun:test";
 
 import {
   buildForkedSessionUiState,
+  resolveRequestedStreamSessionId,
 } from "../../src/routes/chat.js";
 import {
   buildPromptWithLearningContext,
@@ -14,6 +15,38 @@ import { type SessionTurnRecord } from "../../src/sessions/transcriptStore.js";
 import { type MemoryStore } from "../../src/memory/store.js";
 import { type OpencodeRuntimeAdapter } from "../../src/runtime/opencode.js";
 
+describe("resolveRequestedStreamSessionId", () => {
+  it("uses only explicit session_id for normal chat reuse", () => {
+    const createClientSessionId = () => {
+      throw new Error("should not create a demo session id");
+    };
+
+    expect(
+      resolveRequestedStreamSessionId({
+        sessionId: " existing-session ",
+        promptText: "检查压力异常",
+        createClientSessionId,
+      }),
+    ).toBe("existing-session");
+
+    expect(
+      resolveRequestedStreamSessionId({
+        promptText: "检查压力异常",
+        createClientSessionId,
+      }),
+    ).toBeUndefined();
+  });
+
+  it("keeps the pressure trend demo on its generated client session id", () => {
+    expect(
+      resolveRequestedStreamSessionId({
+        promptText: "展示最近压力趋势",
+        createClientSessionId: () => "demo-session",
+      }),
+    ).toBe("demo-session");
+  });
+});
+
 describe("shouldGenerateSessionTitle", () => {
   it("allows auto-title generation for the first turn when the title was not edited", () => {
     expect(