refactor(samples): modernize syntax in youtube playlist.

[abdulalim110]

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Description

This PR refactors samples/youtube/playlist.js to improve code consistency and readability by adopting modern ES6+ standards.

• Refactored string concatenation to use template literals.

• Implemented object property shorthand for the headers object.

• Ensured consistency with existing modern syntax used in other parts of the same file.

Impact

This is a non-breaking change that improves the quality of the sample code. It provides a better "Developer Experience" (DX) for users who refer to these samples to learn the API.

Testing

• Manual verification of the code structure.

• Ran npm run fix to ensure compliance with Google's style guide (GTS/ESLint).

Additional Information

The changes are isolated to the samples/ directory and do not affect the core library logic or auto-generated API files.

Checklist

• Make sure to open an issue before writing your code!
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added
• No new warnings or issues will be generated from this change

Fixes #3883 🦕

[quirogas]

Hi @abdulalim110, thanks for your contribution! However, this is not the right place to make this change, as all these files are autogenerated, and we are not planning to update these templates to ES6 syntax at the moment.