[TypeDeclaration] Add PrivateMethodReturnTypeFromStrictNewArrayRector, split private methods out

[TomasVotruba]

Splits private-method handling out of ReturnTypeFromStrictNewArrayRector into a dedicated, safer rule.

Private methods have no external contract (can't be overridden), so they're the safest case for adding a strict array return type. The new rule runs above the original in the type-declaration level order, and the original now skips private methods.

New rule: PrivateMethodReturnTypeFromStrictNewArrayRector

Handles private methods only. Drops the ClassMethodReturnTypeOverrideGuard check the original needs, since private methods can't be overridden.

 final class SomeClass
 {
-    private function run()
+    private function run(): array
     {
         $values = [];

         return $values;
     }
 }

Changed: ReturnTypeFromStrictNewArrayRector

Now skips private methods (handled by the new rule above); still covers functions and public/protected methods.

 private function shouldSkip(ClassMethod|Function_|Closure $node, Scope $scope): bool
 {
     if ($node->returnType instanceof Node) {
         return true;
     }

+    // private methods are handled by PrivateMethodReturnTypeFromStrictNewArrayRector
+    if ($node instanceof ClassMethod && $node->isPrivate()) {
+        return true;
+    }
+
     return $node instanceof ClassMethod && $this->classMethodReturnTypeOverrideGuard->shouldSkipClassMethod(...);
 }

New rule registered directly above the original in TypeDeclarationLevel.