fix(qqofficial): send complete stream snapshots

[QuasarRyan]

概述 / Overview

请在此部分填写你实现/解决/优化的内容:
Summary of what you implemented/solved/optimized:
#2454

• 在 QQ Official 流式接口的 replace 模式下，每次更新均发送完整的累计文本。
• sent_length 仅用于跳过没有新增内容的非最终更新，不再用于截断发送内容。
• 新增单元测试，确保后续更新仍发送完整文本快照。

更改前后对比截图 / Screenshots

请在此部分粘贴更改前后对比截图（可以是界面截图、控制台输出、对话截图等）:
Please paste the screenshots of changes before and after here (can be interface screenshots, console output, conversation screenshots, etc.):

修改前 / Before:
详见 issue #2454
修改后 / After:

default.mov

检查清单 / Checklist

PR 作者完成 / For PR author

请在方括号间写x以打勾 / Please tick the box with x

• 阅读仓库贡献指引了吗？ / Have you read the contribution guide?
• 我已签署或将在机器人提示后签署 CLA。 / I have signed, or will sign when prompted by the bot, the CLA.
• 与项目所有者沟通过了吗？ / Have you communicated with the project maintainer?
• 我确定已自行测试所作的更改，确保功能符合预期。 / I have tested the changes and ensured they work as expected.

项目维护者完成 / For project maintainer

• 相关 issues 链接了吗？ / Have you linked the related issues?
• 配置项写好了吗？迁移写好了吗？生效了吗？ / Have you written the configuration items? Have you written the migration? Has it taken effect?
• 依赖加到 pyproject.toml 和 core/bootutils/deps.py 了吗 / Have you added the dependencies to pyproject.toml and core/bootutils/deps.py?
• 文档编写了吗？ / Have you written the documentation?

[github-actions]

All contributors have signed the CLA. ✅ 所有贡献者均已签署 CLA。
_{Posted by the CLA Assistant Lite bot.}

[QuasarRyan]

I have read the CLA Document and I hereby sign the CLA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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