[LiveComponent] Support UnionTypes for LiveProps

[sneakyvv]

Problem

Call to undefined method ReflectionUnionType::getName()

Cause

https://github.com/symfony/ux/blob/2.x/src/LiveComponent/src/Metadata/LiveComponentMetadataFactory.php#L77 assuming to work with a ReflectionNamedType.

Reproduce

Create a LiveComponent with

#[LiveProp] public int|string $someProperty;

Use it somewhere

Expected behavior

Either:

1. Support using UnionTypes for LiveProps
2. Throw clear error that UnionTypes are not supported

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I want to contribute, but am not sure if option 1 or 2 is desired. The current $metadatas array is keyed by the property name and is flat, i.e. does not allow multiple types for one property (nor does the LivePropMetadata object), so allowing union types (option 1) will require some changes.

[kbond]

Support using UnionTypes for LiveProps

I definitely prefer this but yeah, would require some work.

[weaverryan]

The types are also used during the dehydration process, so this gets tricky. Even with this simple situation of int|string, if the user enters 2 into a box (which would be entered and, iirc, sent to the server as a string), do so cast that to an int? Or do we only cast of it doesn't match one of the types in the union?

For more complex things, I think it can't be allowed - e.g. to choose a wild example - public Product|int $product. In that case, would se use the entity hydrator to query for the Product or leave it as an int?

[sneakyvv]

Implementing a custom mount() method and transforming the string value into an object is also always an option when some non-standard logic needs to be done, for example when working with Value Objects or other non-Doctrine-entity objects.

So I can agree that it can get complicated pretty fast, which I think means option 1 is out of the window?
I'll create a PR to throw a more helpful error when using union types.