fix(api): handle omission markers in queued WebSocket events (#3944)

## Summary

Fixes serialization errors when WebSocket events containing `omit` or
`not_given` are queued before the connection opens.

## Changes

- Support omission markers in dictionary events queued by synchronous
and asynchronous WebSocket connection managers.

Co-authored-by: marcuswood-oai <323319819+marcuswood-oai@users.noreply.github.com>