feat(api): Add obfuscation field to ChatCompletionChunk (#3690)

## Summary

Introduce an optional obfuscation field on ChatCompletionChunk to
support size-normalized streamed chunks. The new field is optional and
omitted when stream_options.include_obfuscation is false.

## Changes

- Add ChatCompletionChunk.obfuscation: Optional[str]. This optional
string may be included on streamed moderation chunks to normalize chunk
sizes; it is returned by default and omitted when
stream_options.include_obfuscation is false.

Co-authored-by: apcha-oai <228803254+apcha-oai@users.noreply.github.com>