chore: sync tests from upstream.

[He-Pin]

Fix Summary

Root Cause: The JS/Native/WASM platforms utilize the scala-yaml library; specifically, its parseYaml function does not support two features found in YAML multi-document streams:

1. --- <content> — Content immediately following the document marker (e.g., --- > for a folded scalar, or --- 3 for a scalar value).
2. Empty documents (where no content appears after the --- marker until the next ---).

Fix Strategy (3 Changes):

1. src-native/Platform.scala + src-js/Platform.scala: Replaced the manual splitYamlDocuments implementation with parseManyYamls to correctly handle inline content scenarios such as --- > and --- 3. Added a addExplicitNullsForEmptyDocs pre-processing step to insert explicit null text for empty documents, thereby bypassing scala-yaml's limitation regarding the handling of empty documents.
2. src-js/Platform.scala (nodeToJson function): Added a match case for None (specifically case null | None => ujson.Null) to ensure consistency with the Native platform. scala-yaml returns None when decoding null scalars, a scenario that was previously unhandled on the JS platform.
3. test/src/ParseYamlTests.scala: Added regression tests to cover scenarios involving --- 3, --- >, empty documents, bare document markers, and similar cases.