Bug: Product API Accepts Negative Quantity `POST /api/v1/products`

[sagarkumar-webkul]

🔍 Description:

The API currently allows the creation of products with negative quantity values, which is logically invalid and can lead to inconsistent inventory and sales behavior.

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🧪 Steps to Reproduce:

1. Send a POST request to /api/v1/products
2. Use the following JSON body:

{
  "name": "Test Product",
  "description": "Negative quantity test",
  "sku": "NEG-001",
  "quantity": -45,
  "price": 100,
  "entity_type": "products"
}

)