[phpstan] Add PHPStan rule to flag same class-type union in param and return

[TomasVotruba]

Adds PreferGenericTemplateOnSameUnionTypeRule under utils/phpstan, registered in phpstan.neon, to catch the shape generalized in #8584: a method whose single param and return share the same class-type union, e.g.

private function processNullableInstance(BooleanAnd|BooleanOr $node): null|BooleanAnd|BooleanOr

Such a method almost always returns the node it was passed, so a @template generic narrows the return to the concrete passed type and lets callers drop /** @var ... */ hacks.

The rule only fires when every union member is a class type (a Name node). Scalar unions like int|float and mixed unions like Node|array are skipped, since there the return is usually a fresh value, not the input - adding a template would be unsound. Methods that already carry @template are skipped too.

Covered by PreferGenericTemplateOnSameUnionTypeRuleTest with a failing fixture (bare union) and a passing one (already generic, scalar union, mismatched union).

Also applies the @template to the 13 existing matches across the tree, so a full composer phpstan run reports no hits outside the rule test fixtures.