fix(items): correct error type handling and add swagger documentation

- Fix error type mismatch: change 'ITEM.NAME.ALREADY.EXISTS' to 'ITEM_NAME_EXISTS'
- Add ItemErrorType constant with UpperCamelCase keys for better maintainability
- Update all error checks to use the new ItemErrorType constant
- Add ItemErrorResponse.dto.ts with documented error types for swagger
- Add @ApiResponse decorators to document 400 validation errors in swagger

packages/server/src/modules/Items/dtos/ItemErrorResponse.dto.ts

@@ -0,0 +1,112 @@
+import { ApiProperty } from '@nestjs/swagger';
+
+/**
+ * Item API Error Types
+ * These error types are returned when item operations fail validation
+ */
+export enum ItemErrorType {
+  /** Item name already exists in the system */
+  ItemNameExists = 'ITEM_NAME_EXISTS',
+  
+  /** Item category was not found */
+  ItemCategoryNotFound = 'ITEM_CATEOGRY_NOT_FOUND',
+  
+  /** Cost account is not a Cost of Goods Sold account */
+  CostAccountNotCogs = 'COST_ACCOUNT_NOT_COGS',
+  
+  /** Cost account was not found */
+  CostAccountNotFound = 'COST_ACCOUNT_NOT_FOUMD',
+  
+  /** Sell account was not found */
+  SellAccountNotFound = 'SELL_ACCOUNT_NOT_FOUND',
+  
+  /** Sell account is not an income account */
+  SellAccountNotIncome = 'SELL_ACCOUNT_NOT_INCOME',
+  
+  /** Inventory account was not found */
+  InventoryAccountNotFound = 'INVENTORY_ACCOUNT_NOT_FOUND',
+  
+  /** Account is not an inventory type account */
+  InventoryAccountNotInventory = 'INVENTORY_ACCOUNT_NOT_INVENTORY',
+  
+  /** Multiple items have associated transactions */
+  ItemsHaveAssociatedTransactions = 'ITEMS_HAVE_ASSOCIATED_TRANSACTIONS',
+  
+  /** Item has associated transactions (singular) */
+  ItemHasAssociatedTransactions = 'ITEM_HAS_ASSOCIATED_TRANSACTINS',
+  
+  /** Item has associated inventory adjustments */
+  ItemHasAssociatedInventoryAdjustment = 'ITEM_HAS_ASSOCIATED_INVENTORY_ADJUSTMENT',
+  
+  /** Cannot change item type to inventory */
+  ItemCannotChangeInventoryType = 'ITEM_CANNOT_CHANGE_INVENTORY_TYPE',
+  
+  /** Cannot change type when item has transactions */
+  TypeCannotChangeWithItemHasTransactions = 'TYPE_CANNOT_CHANGE_WITH_ITEM_HAS_TRANSACTIONS',
+  
+  /** Inventory account cannot be modified */
+  InventoryAccountCannotModified = 'INVENTORY_ACCOUNT_CANNOT_MODIFIED',
+  
+  /** Purchase tax rate was not found */
+  PurchaseTaxRateNotFound = 'PURCHASE_TAX_RATE_NOT_FOUND',
+  
+  /** Sell tax rate was not found */
+  SellTaxRateNotFound = 'SELL_TAX_RATE_NOT_FOUND',
+  
+  /** Income account is required for sellable items */
+  IncomeAccountRequiredWithSellableItem = 'INCOME_ACCOUNT_REQUIRED_WITH_SELLABLE_ITEM',
+  
+  /** Cost account is required for purchasable items */
+  CostAccountRequiredWithPurchasableItem = 'COST_ACCOUNT_REQUIRED_WITH_PURCHASABLE_ITEM',
+  
+  /** Item not found */
+  NotFound = 'NOT_FOUND',
+  
+  /** Items not found */
+  ItemsNotFound = 'ITEMS_NOT_FOUND',
+}
+
+/**
+ * Item API Error Response
+ * Returned when an item operation fails
+ */
+export class ItemErrorResponseDto {
+  @ApiProperty({
+    description: 'HTTP status code',
+    example: 400,
+  })
+  statusCode: number;
+
+  @ApiProperty({
+    description: 'Error type identifier',
+    enum: ItemErrorType,
+    example: ItemErrorType.ItemNameExists,
+  })
+  type: ItemErrorType;
+
+  @ApiProperty({
+    description: 'Human-readable error message',
+    example: 'The item name is already exist.',
+    required: false,
+    nullable: true,
+  })
+  message: string | null;
+
+  @ApiProperty({
+    description: 'Additional error payload data',
+    required: false,
+    nullable: true,
+  })
+  payload: any;
+}
+
+/**
+ * Item API Error Response Wrapper
+ */
+export class ItemApiErrorResponseDto {
+  @ApiProperty({
+    description: 'Array of error details',
+    type: [ItemErrorResponseDto],
+  })
+  errors: ItemErrorResponseDto[];
+}