Incorrect validation message for multi-type schemas

[araffass]

When validating schemas with multiple types, such as:

"type": ["string", "null"]

the generated validation error message may be incorrect.

Example current output:

expected type: one of , , , , actual: integer

This makes it difficult for downstream libraries to display meaningful validation errors.

Expected output:

expected type: [string, null], actual: integer

This PR updates the multi-type validation failure message to correctly display the list of expected types.