support for untagged in DocumentDecoderSchemaVisitor

[lewisjkl]

There may be a reason why we didn't have this before, but we came across a case that seems to indicate this was overlooked. We already had support for encoding untagged unions from documents, but not decoding them. Feel free to close this if I missed something and there actually is a good reason to omit them.

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

[daddykotex]

Again, surprising we just find out that we did not support this!

thanks for the fix