fix: use qwen-flash to rename

[xinquiry]

变更内容

• 新功能
• 修复 Bug
• 增强重构
• 其他（请描述）

简要描述本次 PR 的主要变更内容。

相关 Issue

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• 已在本地测试通过
• 已补充/更新相关文档
• 已添加测试用例
• 代码风格已经过 pre-commit 钩子检查

其他说明

如有特殊说明或注意事项，请补充。

Summary by Sourcery

增强功能：

• 将 QWEN 标题生成从使用 qwen3-0.6b 模型切换为使用 qwen-flash 模型。

Original summary in English

Summary by Sourcery

Enhancements:

• Switch QWEN title generation to use the qwen-flash model instead of qwen3-0.6b.

[sourcery-ai]

评审者指南（在小型 PR 上折叠）

评审者指南

将 QWEN 提供方的标题生成模型从 qwen3-0.6b 更新为 qwen-flash，使主题生成与新模型的选择保持一致，同时保持其他提供方不变。

文件级变更

变更	详情	文件
将 QWEN 的标题生成模型切换为使用 qwen-flash 变体。	在标题生成模型选择的辅助函数中，当提供方类型为 QWEN 时，将返回的模型标识从 qwen3-0.6b 更新为 qwen-flash。 保持其他提供方类型的选择逻辑，以及对 session_model 或 default_model 的回退逻辑不变。	service/app/core/chat/topic_generator.py

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Original review guide in English

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Updates the QWEN provider’s title generation model selection to use the qwen-flash model instead of qwen3-0.6b, aligning topic generation with the new model choice while leaving other providers unchanged.

File-Level Changes

Change	Details	Files
Switch QWEN title generation model to use the qwen-flash variant.	Update the model identifier returned when the provider type is QWEN from qwen3-0.6b to qwen-flash in the title generation model selection helper. Keep the selection logic for other provider types and the fallback to session_model or default_model unchanged.	service/app/core/chat/topic_generator.py

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[sourcery-ai]

Hey - 我在这里给出了一些整体反馈：

• 请检查配置、路由或模型能力映射中是否还有对 qwen3-0.6b 的残留引用。由于现在用于 QWEN 标题生成的模型是 qwen-flash，需要避免因此产生不匹配。
• 如果 qwen-flash 在 token 上限或性能特性方面与 qwen3-0.6b 不同，建议验证在长对话场景下，标题生成的提示词和截断逻辑是否仍然按预期工作。

面向 AI 代理的提示

Please address the comments from this code review:

## Overall Comments
- Consider checking for any remaining references to `qwen3-0.6b` in configuration, routing, or model capability mappings to avoid mismatches now that the QWEN title generation model is `qwen-flash`.
- If `qwen-flash` has different token limits or performance characteristics than `qwen3-0.6b`, it may be worth validating that the title generation prompts and truncation logic still behave as expected for longer conversations.

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Original comment in English

Hey - I've left some high level feedback:

• Consider checking for any remaining references to qwen3-0.6b in configuration, routing, or model capability mappings to avoid mismatches now that the QWEN title generation model is qwen-flash.
• If qwen-flash has different token limits or performance characteristics than qwen3-0.6b, it may be worth validating that the title generation prompts and truncation logic still behave as expected for longer conversations.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider checking for any remaining references to `qwen3-0.6b` in configuration, routing, or model capability mappings to avoid mismatches now that the QWEN title generation model is `qwen-flash`.
- If `qwen-flash` has different token limits or performance characteristics than `qwen3-0.6b`, it may be worth validating that the title generation prompts and truncation logic still behave as expected for longer conversations.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
service/app/core/chat/topic_generator.py	0.00%	1 Missing ⚠️

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