fix(functions): migrate imagemagick sample to sharp and improve test stability

[angelcaamal]

Description

Fixes Internal: b/503436089

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Code Review

This pull request migrates the image blurring function from ImageMagick (using the gm library) to the sharp library. The changes include updating the dependencies in package.json, refactoring the core logic in index.js to handle temporary files with sharp, and enhancing the test suite with better error handling. The review feedback correctly identifies that the engines field in package.json must be updated to support sharp's Node.js requirements. Additionally, there are several recommendations to improve code quality by removing dead code (commented-out lines), fixing inconsistent language in comments, and ensuring consistent async/await usage during file cleanup.