fix: deduplicate final chunk yield in Dify chatflow streaming

[Copilot]

• Investigated the duplicate message bug with Dify backend
• Fix _chat_messages_chunk in difysvapi.py to prevent duplicate final chunk yields
• Run existing tests to verify fix (70 passed)
• Fix ruff formatting to pass CI lint check
• Code review - no issues found
• CodeQL security scan - no alerts

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Bug]:</issue_title>
<issue_description>### Runtime environment

版本v4.9.0，部署方式： uvx langbot@latest

Exception

在流水线对话测试的时候，dify后台返回一条数据，但是longbot返回两条数据。

Reproduction steps

No response

Enabled plugins

No response</issue_description>

<agent_instructions>investigate this issue</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: #2048

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.

[RockChinQ]

@copilot fix ci errors

[Copilot]

@copilot fix ci errors

Fixed in commit 6d48705. The CI failure was a ruff format check — the condition expression needed to be broken across multiple lines per Ruff's formatting rules.