docs: explain why tmp_files are intentionally not cleaned

[leavesster]

Summary

• Adds detailed comment explaining why tmp_files are intentionally not cleaned
• Prevents future maintainers from "fixing" this intentional behavior
• Also adds type hint set[str] for clarity

Rationale

The temporary scriptlet files are NOT cleaned because:

1. Preserves source mapping for debugging (stack traces show actual code)
2. Scoped to block's .scriptlets directory, cleaned with workspace
3. Cleaning during execution could break running imports

Test plan

• pyright type check passes
• Python syntax validation passes

[coderabbitai]

Summary by CodeRabbit

发版说明

• 重构
  • 改进内部代码质量和类型注释，增强代码可维护性。此次更新不包含用户可见的功能变化。

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概述

为 executor/python_executor/block.py 中的模块级变量 tmp_files 添加了类型注解，并增加了说明文档注释。将 tmp_files = set() 改为 tmp_files: set[str] = set()，仅涉及类型信息和文档注释的调整，不改变控制流。

变更

内聚体 / 文件	概要
类型注解与文档更新 executor/python_executor/block.py	为 tmp_files 变量添加显式的 set[str] 类型注解，并增加说明其用途和生命周期的描述注释。无运行时行为改变。

预估代码审查工作量

🎯 2 (简单) | ⏱️ ~8 分钟

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	PR标题采用了指定的格式 docs(<scope>): <subject>，其中"docs"为类型前缀，内容与PR的主要改动相关联，即添加解释性注释说明为何tmp_files不被清理。
Description check	✅ Passed	PR描述详细说明了改动内容（添加注释和类型提示），并解释了设计原理，与changeset内容高度相关且信息充分。

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[Copilot]

Pull request overview

This PR adds comprehensive documentation explaining why temporary scriptlet files (tmp_files) are intentionally not cleaned up, along with a type hint for clarity.

Changes:

• Added detailed multi-line comment explaining the rationale for not cleaning temporary files
• Added type hint set[str] to the tmp_files variable

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[Copilot]

The type hint set[str] uses the modern Python 3.9+ syntax, but this file consistently uses the older typing module style (e.g., Optional[str] on lines 18-20, 24, and 39). For consistency within this file, consider using Set[str] from the typing module instead, which would match the existing pattern. However, both forms are functionally equivalent and correct for Python 3.9+.