Make sure provider methods are callable in Generator::getFormatter()

[max-m]

What is the reason for this PR?

Currently non-public methods of providers can override public methods of providers below them in the chain.

If accepted, this change can also be backported to 1.x.

• A new feature
• Fixed an issue

Author's checklist

• Follow the Contribution Guide
• New features and changes are documented

Note: Before and after the change, the same 7 test cases failed.

Summary of changes

class ProviderA extends \Faker\Provider\Base {
    public function testFN() {
        echo 'Hello, world!';
    }
}

class ProviderB extends \Faker\Provider\Base {
    private function testFN() {
        echo 'Get off my lawn!';
    }
}

$generator = \Faker\Factory::create();
$generator->addProvider(new ProviderA($generator));
$generator->addProvider(new ProviderB($generator));
$generator->testFN();

This example will throw a fatal error:
call_user_func_array(): Argument #1 ($callback) must be a valid callback, cannot access private method ProviderB::testFN()

If we take a look at the Generator class, we find

Faker/src/Generator.php

Lines 723 to 729 in 36ab451

	foreach ($this->providers as $provider) {
	if (method_exists($provider, $format)) {
	$this->formatters[$format] = [$provider, $format];
	return $this->formatters[$format];
	}
	}

method_exists() also returns true for protected and private methods.
See https://3v4l.org/eXkna

Instead we can use is_callable(), which respects the visibility, see https://3v4l.org/rHeUg

With this change we get Hello, world! as a result. :)

Review checklist

• All checks have passed
• Changes are added to the CHANGELOG.md
• Changes are approved by maintainer

[max-m]

I have added a couple of simple tests, but had to comment two of them out because of the protected_to_private CS fixer rule.

vendor/bin/phpunit --filter "Generator"
PHPUnit 9.6.22 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.7
Configuration: Faker/phpunit.xml.dist
Random Seed:   1740756979

Testing
......................................38 / 38 (100%)

Time: 00:00.040, Memory: 8.00 MB

OK (38 tests, 49 assertions)

Legacy deprecation notices (7)

This run included the testProtectedProviderMethodsAreNotCalled() and testProtectedProviderMethodsDoNotShadowPublicMethods() tests to make sure they work as expected.