[Refactor] Use declarative array methods in filterCustomHeaders

[gonzaloriestra]

WHY are these changes introduced?

Improves code readability and maintainability by using declarative array methods in filterCustomHeaders.

WHAT is this pull request doing?

Refactored filterCustomHeaders in packages/cli-kit/src/public/node/graphiql/utilities.ts to replace the imperative for...of loop with declarative Object.entries().filter() and Object.fromEntries().

How to test your changes?

CI

Post-release steps

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 18138345370755277611 started by @gonzaloriestra

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