[fix]: Null Handling for getStockById

Description

• Detailed Description:
  The method getStockById in OperationController returns null when no stock is found, which is not an informative response for API consumers. A proper HTTP 404 Not Found should be returned.
• How to Solve:
  1. Use ResponseEntity:
     Modify the method to return a ResponseEntity so that you can control the HTTP status.
  2. Check for Null:
     If the stock is null, return a 404 response:

     @GetMapping("/{id}")
     public ResponseEntity<Stock> getStockById(@PathVariable Long id) {
         Stock stock = operationService.getStockById(id);
         return (stock != null) ? ResponseEntity.ok(stock) : ResponseEntity.notFound().build();
     }
     

  3. Test the Endpoint:
     Use tools like Postman or cURL to ensure that a non-existent stock ID returns a 404 status.

Start in OperationController at the getStockById endpoint and inspect how it calls operationService.getStockById. Make the endpoint return HTTP 404 when no stock is found and verify the behavior for an existing and a nonexistent ID with Postman or cURL.