[Refactor] Optimize copyDirectoryContents with glob cwd

[gonzaloriestra]

WHY are these changes introduced?

This refactor improves the performance and readability of the copyDirectoryContents utility. By using the cwd option in the glob call, we avoid brittle manual string manipulation to calculate relative paths, which aligns with the project's performance guidelines for file-intensive operations.

WHAT is this pull request doing?

• Updated copyDirectoryContents in packages/cli-kit/src/public/node/fs.ts to use glob('**/*', {cwd: srcDir}).
• Replaced the manual for...of loop with a more idiomatic .map() and Promise.all() approach.
• Removed manual regex-based path stripping.

How to test your changes?

Run the unit tests for the affected package:

pnpm --filter @shopify/cli-kit vitest run src/public/node/fs.test.ts

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing

Duplicate check

Query: git log --grep="Refactor" --pretty=format:"%h %s" -n 50

No previous PRs were found that address this specific optimization in the copyDirectoryContents function within cli-kit.

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PR created automatically by Jules for task 7992711792130112196 started by @gonzaloriestra

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