Fix None issue in reorganizer queue

[J1awei-Yang]

Description

Summary: (summary)
fix the None type in minheap of reorganizer.

Reviewer: @CaralHsi

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have added necessary documentation (if applicable) | 我已添加必要的文档（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人

[Copilot]

Pull Request Overview

This PR fixes a None-handling issue in the reorganizer queue by replacing the use of None as a sentinel value with a proper "end" operation type. The change improves type safety and makes the queue termination mechanism more explicit.

• Introduces a new "end" operation type to the QueueMessage class
• Replaces None sentinel value with QueueMessage(op="end") for queue termination
• Improves null safety in message preprocessing by adding additional None checks

[Copilot]

The import statement for 'collections' has been moved to the top but the old import on line 9 wasn't removed, creating a duplicate import. Remove the duplicate import statement.

[Copilot]

The op_priority dictionary is missing the 'update' operation which is defined in the Literal type. This will cause a KeyError when comparing QueueMessage objects with op='update'.

Suggested change

	op_priority = {"add": 2, "remove": 2, "merge": 1, "end": 0}
	op_priority = {"add": 2, "remove": 2, "merge": 1, "update": 1, "end": 0}