fix(livestream): stop all channels before pool update to avoid FAILED_PRECONDITION

[angelcaamal]

Description

Fixes Internal: b/511248282

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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

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[gemini-code-assist]

Code Review

This pull request modifies the test cleanup logic in livestream.test.js to attempt stopping all channels before checking their age. The review feedback suggests filtering channels by a name prefix to prevent disrupting other test runs and refining the error handling to check for specific error codes when a channel fails to stop.