[Refactor] Use flatMap in LinesDiffContentToken

[gonzaloriestra]

WHY are these changes introduced?

Modernizes the code by using the idiomatic .flatMap() method instead of chaining .map().flat(). This improves readability and follows modern JavaScript patterns.

WHAT is this pull request doing?

• Refactors LinesDiffContentToken.output() in @shopify/cli-kit to use .flatMap().
• Adds unit tests for LinesDiffContentToken in content-tokens.test.ts to ensure correct behavior.
• Cleans up and consolidates imports in content-tokens.test.ts.

How to test your changes?

CI

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 — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 16588474565286965260 started by @gonzaloriestra

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