[php] fixed model_enum.mustache for real usage

Created by: 77web

fixed Enum usage in PHP APIClients. I tested actual API requests with this new EnumInterface, which performs API requests successfully 😄

before

// code which causes error in static analysis
$someModel = new SomeModel();
$someModel->setSomeEnumField(SomeEnum::VALUE_A); // error, setter has type declaration in phpdoc with "SomeEnum" , SomeEnum::VALUE_A is not a "SomeEnum" instance, but just a string in php

// code to avoid error
$someModel = new SomeModel([
    'some_enum_field' => SomeEnum::VALUE_A,
]);

after

$someModel = new SomeModel();
$someModel->setSomeEnumField(new SomeEnum(SomeEnum::VALUE_A));

note: php8.1 has native Enum feature and it allows code like this.

$someModel = new SomeModel();
$someModel->setSomeEnumField(SomeEnum::VALUE_A);

But, many projects have not yet updated their environments to php8.1.

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