[Refactor] Build cookie record declaratively in parseCookies

[github-actions]

WHY are these changes introduced?

parseCookies builds its result by declaring an empty record and mutating it inside a forEach, while its sibling serializeCookies in the same file is already written declaratively. The imperative accumulator makes the reader track mutation across the callback to understand what the function returns.

WHAT is this pull request doing?

Rewrites parseCookies to build the record with flatMap and Object.fromEntries, so the key/value pairs are produced as data and the record is constructed in one expression. Destructuring the regex match replaces the positional parts[1]/parts[2] indexing, and the return type is now stated explicitly.

Behavior is unchanged: entries without a key are still skipped, keys and values are still trimmed, a missing value still becomes an empty string, and repeated keys still resolve to the last occurrence.

How to manually test your changes?

shopify theme dev

Browse the previewed storefront and confirm the session is preserved across requests (no repeated login or lost preview theme).

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