chore(cart): remove redundant gift card code deduplication

[kdaviduik]

WHY are these changes introduced?

E2E testing confirmed the Storefront API handles duplicate gift card codes gracefully (idempotent behavior). The client-side filtering in cartGiftCardCodesUpdate was redundant overhead that doesn't align with Hydrogen's thin wrapper pattern.

WHAT is this pull request doing?

Removes the uniqueCodes filtering logic from cartGiftCardCodeUpdateDefault.ts.

Behavior change: None. The API deduplicates identically to the client-side logic that was removed.

Scenario	API Behavior
Duplicate code in same call	Idempotent - applied once, no error
Re-applying already applied code	Idempotent - no error, no duplicate

HOW to test your changes?

This is an internal cleanup with no observable behavior change. The existing tests verify the mutation still works correctly.

// This still works exactly the same:
await cart.updateGiftCardCodes(['CODE1', 'CODE1', 'CODE2']);
// Result: CODE1 and CODE2 applied (API handles deduplication)

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
  • No changeset needed - internal cleanup with no user-facing impact
• I've added tests to cover my changes
• I've added or updated the documentation

[kdaviduik]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• chore(cart): remove redundant gift card code deduplication #3402 👈 (View in Graphite)
• feat(cart): add cartGiftCardCodesAdd mutation #3401
• update SFAPI and CAAPI to 2025-10 #3352 : 4 other dependent PRs (#3284 , #3393 , #3398 and 1 other)
• main

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[shopify]

Oxygen deployed a preview of your kd-2025-10-remove-redundant-reduplication branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 23, 2026 4:46 AM

Learn more about Hydrogen's GitHub integration.

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run npm run changeset add to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.