laravel validation does not function as expected

I'm trying to create a RESTful API using Laravel 10,

when trying to validate the inputs, validation does not work as expected and I get database error for the unique field (obviously when I try to add the value which already exists in db) instead of the proper response provided by the validation process.

here's routes/api.php file:

Route::controller(AuthController::class)->group(function () {
    Route::post('register', 'register');
});

here's the app/Controller/AuthController.php file:

public function register(RegisterRequest $request) {
    /**
     * some code to register the user
     */
    $user = new User;
    $user->mobile_no = request('mobile')
}

here's the app/Requests/RegisterRequest.php file:

public function rules(): array
{
    return [
        'mobile'=>'required|unique:users,mobile_no|ir_mobile:zero',
    ];
}

public function messages() {
    return [
        'mobile.unique' => 'a custom msg here',
    ];
}

public function failedValidation(Validator $validator)
{
    // the following function exists in `app/helpers.php` and works currectly
    throwBadRequestError([
        'errors' => $validator->errors(),
        'request' => $_REQUEST
    ]);
}

here's a screenshot of postman:

why does not the validation throw an error instead of database?

many thanks in advance.

From SQL error message the column you are updating is users.mobile while you are testing users.mobile_no in the validation