[Bug]: 错误的创建新会话

## QwenPaw Version

主干：截止到a403b2433af6ac74404d69fc80a289658996226e提交

## Description

1、点击新建任务，创建一个新会话，然后随便问一句，等待AI回答完成，此时历史会话里会多一个新增的会话
2、再次点击新建任务，然后点击上一步自动创建出来的会话，并且在这个会话里进行第二次提问
3、会发现侧边栏又创建了一个会话，而不是延续在刚才的会话里

**Related PR(s):** #(optional)

**Security considerations:** [If applicable, e.g. auth, env/config exposure]

## Component(s) Affected

- [√] Core / Backend (app, agents, config, providers, utils, local_models)
- [√] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Environment

- **QwenPaw version:** [a403b2433af6ac74404d69fc80a289658996226e git commit]
- **OS:** [ Windows 10 ]
- **Install method:** [桌面APP安装]
- **Python version (if applicable):** [e.g. 3.10]

## Steps to Reproduce
如上所示
## Actual vs Expected

- **Actual:** 创建新会话
- **Expected:** 延续点击的会话

## Logs / Screenshots

[Paste relevant log output or attach screenshots. Use code blocks for logs.]

```
(paste logs here)
```

## Additional Notes

[Optional: workarounds, similar issues, etc.]

Reproduce the reported flow first (create task, ask, then create task again and return to the previous session for a second question) to confirm the extra session creation. Then inspect the Console code paths for the New Task action and session selection, and follow the request sent to the backend session/message API to verify whether the selected session ID is preserved. Next, read the backend session-reuse/create handler involved. Done means the second question in a selected old session continues in that session and the sidebar does not add a new one.

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