fix: YAML, Add deserialization Handler for DeletedKeyWithId

[Eideren]

PR Details

Some definition to ensure the rest makes sense, here's how an entry for an item in a collection looks like in YAML:

Materials:
    4f2a44a83ccd0b9a1d6f8d3f5f0772d2~0: d9db4834-f7ce-4f6a-be87-d54732302ec3:Ground Material

4f2a44a83ccd0b9a1d6f8d3f5f0772d2~0 is a KeyWithId, 4f2a44a83ccd0b9a1d6f8d3f5f0772d2 would be the Id, while 0 is the key.

Deserializing a collections whose items defined on the base prefab have been removed would lead to a warning when loading in a project;

FormatException: The input string '' was not in a correct format.
   at System.Number.ThrowFormatException[TChar](ReadOnlySpan`1 value)
   at System.Int32.Parse(String s, IFormatProvider provider)
   at Stride.Core.Yaml.Serialization.Serializers.PrimitiveSerializer.ConvertFrom(ObjectContext& context, Scalar scalar) in I:\stride\sources\core\Stride.Core.Yaml\Serialization\Serializers\PrimitiveSerializer.cs:line 133
   at Stride.Core.Yaml.KeyWithIdSerializer.ConvertFrom(ObjectContext& objectContext, Scalar fromScalar) in I:\stride\sources\assets\Stride.Core.Assets\Yaml\KeyWithIdSerializer.cs:line 42

It's fairly easy to repro;

• Create a prefab with a model
• Add the prefab to a scene
• Enable the material toggle on the model component of the prefab you added in the scene
• Open the prefab itself and enable the material toggle on that prefab's model component

This is how the two yaml look like after this operation:
Base:

Materials:
    4f2a44a83ccd0b9a1d6f8d3f5f0772d2~0: d9db4834-f7ce-4f6a-be87-d54732302ec3:Ground Material

Derived:

Materials:
    4313c8fdf0379e96de405f63ca8da803*~0: d9db4834-f7ce-4f6a-be87-d54732302ec3:Ground Material
    4f2a44a83ccd0b9a1d6f8d3f5f0772d2~: ~(Deleted)

We might naively think that the deleted entry on the derived one doesn't make a ton of sense, but remember that we started by changing the derived one before changing the base one. Given that specificity, index 0 on the derived one should not inherit any changes from index #0 on its base. That would break user expectations as something overriden on derived should never be affected by changes on the base.

What's going on: KeyWithIdSerializer.ConvertFrom() calls PrimitiveSerializer.ConvertFrom() which in turn throws as the keyString provided is empty here

stride/sources/assets/Stride.Core.Assets/Yaml/KeyWithIdSerializer.cs

Lines 40 to 43 in 137215c

	var context = new ObjectContext(objectContext.SerializerContext, null, keyDescriptor);
	var key = scalarKeySerializer.ConvertFrom(ref context, new Scalar(keyString));
	var result = Activator.CreateInstance(typeof(KeyWithId<>).MakeGenericType(keyType), id, key);

The ConvertTo method of this same class does not output a key when the value is a removal;

stride/sources/assets/Stride.Core.Assets/Yaml/KeyWithIdSerializer.cs

Lines 66 to 71 in 137215c

	objectContext.Instance = key;
	if (key.IsDeleted)
	return $"{itemIdPart}~";
	var keyString = scalarKeySerializer.ConvertTo(ref context);

Looks like that particular implementation wasn't entirely finished, so I added in logic in ConvertFrom to mirror ConvertTo's behavior.

Related Issue

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.