Prevent creation of duplicated wishlist names (#7906)

[Shreyansh284]

#7906

Summary

Implements backend validation to prevent customers from creating duplicate wishlist names.

Changes

• Added duplicate name check in ShoppingCartController.AddWishlist
• Added localization message Wishlist.DuplicateNameNotAllowed
• Returns proper JSON response on conflict

Testing

• Verified customers cannot create a wishlist with an existing name
• Verified guest and unique name creation still works correctly

Localization

Added to resource file:

<LocaleResource Name="Wishlist.DuplicateNameNotAllowed">
  <Value>A wishlist with this name already exists.</Value>
</LocaleResource>

[Shreyansh284]

Here is how error message looks like

[Shreyansh284]

Hi @AndreiMaz, @DmitriyKulagin 👋
Just following up — please let me know if any changes or improvements are needed on this PR.
Thanks!