[fix]: Null Handling for getStockById

[0PrashantYadav0]

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.