Task: build a API for maintaining a shopping cart in JavaScript using nest.

API specification:

openapi: 3.0.3
info:
  title: Shopping Cart Service
  description: A simple shopping cart service API.
  version: 1.0.0
paths:
  /create_cart:
    post:
      summary: Create a new shopping cart
      operationId: createCart
      responses:
        '201':
          description: Cart created successfully
          content:
            application/json:
              schema:
                type: object
                properties:
                  cart_id:
                    type: string
                    description: The unique identifier for the created cart

  /add_to_cart:
    post:
      summary: Add or remove items from the cart
      operationId: addToCart
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                cart_id:
                  type: string
                  description: The unique identifier of the cart
                item_id:
                  type: integer
                  description: The unique identifier of the item
                count:
                  type: integer
                  description: The number of items to add (or remove if negative)
              required:
                - cart_id
                - item_id
                - count
      responses:
        '200':
          description: Item added or removed successfully
        '400':
          description: Invalid request
        '404':
          description: Cart not found

  /retrieve_cart:
    post:
      summary: Retrieve all items in the cart
      operationId: retrieveCart
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                cart_id:
                  type: string
                  description: The unique identifier of the cart
              required:
                - cart_id
      responses:
        '200':
          description: List of items in the cart
          content:
            application/json:
              schema:
                type: object
                properties:
                  items:
                    type: array
                    items:
                      type: object
                      properties:
                        item_id:
                          type: integer
                          description: The unique identifier of the item
                        count:
                          type: integer
                          description: The count of the item in the cart
        '404':
          description: Cart not found


Scenario instructions:

Environment instructions:

The NestJs was already created using the `nest new` command.

You need to populate the following files:
src/app.controller.spec.ts
src/app.controller.ts
src/app.module.ts
src/app.service.ts
src/main.ts


Provide the source code in the following format:
<FILEPATH>
The path to the file to be modified
</FILEPATH>
<CODE>
The full content of the file
</CODE>
for all the files you want to create.

Example:
<FILEPATH>
path/to/file.py
</FILEPATH>
<CODE>
def hello_world():
    pass
</CODE>



Make sure that the command `npm run start` starts the app successfully.


Allowed packages:

{
  "scripts": {
    "build": "nest build",
    "start": "nest start"
  },
  "engines": {
    "npm": ">=10.0.0",
    "node": ">=20.0.0"
  },
  "dependencies": {
    "@nestjs/common": "11.0.1",
    "@nestjs/core": "11.0.1",
    "@nestjs/platform-express": "11.0.1",
    "reflect-metadata": "0.2.2",
    "rxjs": "7.8.1",
    "bcrypt": "5.1.1",
    "dotenv": "16.4.7",
    "express": "4.21.2",
    "uuid": "11.0.3",
    "sqlite": "5.1.1",
    "sqlite3": "5.1.7",
    "multer": "1.4.5-lts.1",
    "cookie-parser": "1.4.7",
    "jsonwebtoken": "9.0.2"
  },
  "devDependencies": {
    "@nestjs/cli": "11.0.0",
    "@nestjs/schematics": "11.0.0",
    "@swc/cli": "0.6.0",
    "@swc/core": "1.10.8",
    "@types/express": "5.0.0",
    "@types/multer": "1.4.12",
    "@types/node": "22.10.7",
    "prettier": "3.4.2",
    "source-map-support": "0.5.21",
    "supertest": "7.0.0",
    "ts-loader": "9.5.2",
    "ts-node": "10.9.2",
    "typescript": "5.7.3"
  }
}