[PHPStan 2.1.14] Fix resolve __toString() parent non builtin method without return type

[samsonasik]

@TomasVotruba this is the real patch for resolve __toString from non builtin parent method.

This require revert of PR:

• Update fixture for __toString() when parent mixed return, to always add string, as work on DowngradeStringReturnTypeOnToStringRector rector-downgrade-php#272

[samsonasik]

@TomasVotruba fixed and fixture reverted on rectorphp/rector-downgrade-php#273 🎉

[samsonasik]

All checks have passed 🎉 @TomasVotruba I am merging it ;)

[samsonasik]

I created PR to phpstan-src for possible solution there so we can back to original implementation to avoid this hack if accepted on phpstan side:

• Only set return native type from null return type when declaring class is built in on __toString() phpstan/phpstan-src#3978
• Revert "Useful PhpMethodReflection native type refactoring from #3966" phpstan/phpstan-src#3979