fix(plugin-ecommerce): return CartOperationResult and propagate errors in cart operations

[teastudiopl]

Previously, addItem, removeItem, incrementItem, decrementItem and clearCart
swallowed errors in catch blocks and returned void, making it impossible for
consumers to determine whether an operation succeeded or failed.

Changes:

• return CartOperationResult (including refreshed cart) on success
• re-throw errors in catch blocks so consumers can use try/catch
• update defaultContext placeholders to match new return types

This allows consumers to handle cart operation results:

  try {
   const result: CartOperationResult | undefined  = await addItem(newItem)
    // result contains the updated cart and success (true/false) flag
  } catch (error) {
    // handle failure
  }

usage:

    
        try {
          const result = await addItem(newItem)

          console.log(result)

          result?.success ? toast.success(t('addedToCart')) : toast.error(t('failedAddedToCart'))
        } catch (error) {
          toast.error(t('failedAddedToCart'))
        }