fix: preserve chat stream moderation results (#3864)

Preserves moderation results in chat stream snapshots and final
completions, including reports delivered after content. Later omitted or
null fields retain the last report; unmoderated streams remain
unchanged.

Validation: 324 tests passed (14 skipped), plus 19 focused tests with
Pydantic v1. Ruff, mypy, changed-file formatting, and wheel/sdist builds
pass.