chore: update prettier workflow to use pnpm instead of npm

[Copilot]

The project migrated to pnpm (packageManager: "pnpm@10.25.0" in package.json), but the prettier workflow still used npm commands, causing workflow failures.

Changes:

• Added pnpm installation step via pnpm/action-setup@v4 with version 10.25.0
• Replaced npm install → pnpm install
• Replaced npx prettier → pnpm exec prettier
• Updated safety comment to reference pnpm

Original prompt

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This section details on the original issue you should resolve

<issue_title>项目从 npm 改成了 pnpm，因此，prettier workflow 需要更新，否则会执行失败</issue_title>
<issue_description></issue_description>

<agent_instructions>git commit msg 格式遵循项目规范，使用 chore: xxx,或者 feat: xxx 这种，英文</agent_instructions>

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

• Fixes 项目从 npm 改成了 pnpm，因此，prettier workflow 需要更新，否则会执行失败 #4921

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[idoocs]

🤭 感谢你的提交，请检查你的改动是否符合以下项目规范。

1. 格式化

我们项目中各种编程语言代码（包括文档）所采用的格式化工具不同，提交 pr 之前必须确保代码、文档正确格式化。

• .{md,js,ts,php,sql,rs} 采用 prettier
• .{c,cpp,java} 采用 clang-format
• .{py} 采用 black
• .{go} 采用 gofmt
• 其它待完善

2. Git 提交信息

我们项目遵循 AngularJS Git Commit Message Conventions 规范，我们希望你的提交信息尽可能与项目保持一致。

• 新增或修改题解：feat: add/update solution(s) to lc problem(s): No.xxxx
• 修复错误：fix: xxxx
• 日常维护：chore: xxx

3. 其它补充

新增题解及代码时，需要创建 Solution.xxx 源代码文件（如果已存在，请确认算法是否更优，是则覆盖已有算法代码），同时，需要在 README.md 以及 README_EN.md 中添加对应的代码片段（英文文件中不要出现中文注释）
另外，编码风格（比如变量、函数的命名），尽量跟项目已有代码保持一致。

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🤭 Thank you for your contribution. Please check if your changes comply with the following project specifications.

1. Formatting

We use different formatting tools for various programming languages (including documentation) in our project. You must ensure that the code and documentation are correctly formatted before submitting a pr.

• .{md,js,ts,php,sql,rs} use prettier
• .{c,cpp,java} use clang-format
• .{py} use black
• .{go} use gofmt
• Others to be improved

2. Git Commit Message

Our project follows the AngularJS Git Commit Message Conventions. We hope that your submission information is as consistent as possible with the project.

• Add or modify solutions: feat: add/update solution(s) to lc problem(s): No.xxxx
• Fix errors: fix: xxxx
• Routine maintenance: chore: xxx

3. Other notes

When adding solutions and code, you need to create a Solution.xxx source code file (if it already exists, please confirm whether the algorithm is better, if yes, overwrite the existing algorithm code), and at the same time, you need to add the corresponding code snippets in README.md and README_EN.md (do not have Chinese comments in the English file)
In addition, the coding style (such as the naming of variables and functions) should be as consistent as possible with the existing code in the project.

[Copilot]

Pull request overview

This PR updates the prettier workflow to use pnpm instead of npm, aligning with the project's migration to pnpm as specified in package.json (packageManager: "pnpm@10.25.0").

Key Changes:

• Added pnpm installation step using pnpm/action-setup@v4 with version 10.25.0
• Replaced all npm commands with pnpm equivalents (npm install → pnpm install, npx prettier → pnpm exec prettier)
• Updated security comment to reflect pnpm usage

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