Avoid freezing execution results during finalization

[ydah]

Ruby head currently freezes the nested array in Array#flatten!'s single-array fast path:

result = []
[result].flatten!
result.frozen? # => true

GraphQL::Execution::Finalize used flatten! while locating errors in nested list results. This unintentionally froze the execution result array, then null propagation attempted to update it and raised FrozenError.

This change uses non-destructive flatten instead. It preserves the existing recursive traversal without freezing result arrays.

The existing [Test], [Test!], and [Test!]! null-propagation cases fail before this change and pass afterward on Ruby 4.1.0dev. They also pass on Ruby 4.0.

[rmosolgo]

Thanks for tracking down this test failure.

Do you know if this .freeze is intentional or accidental?

With .flatten we're allocating a new Array -- which I'd rather not, if we can help it.