fix(types): type response annotation added event

[pranjalbhatia710]

Summary

• type ResponseOutputTextAnnotationAddedEvent.annotation as the existing Responses Annotation union instead of plain object
• keep event parsing aligned with ResponseOutputText.annotations, so streamed annotation-added events expose typed citation/file-path models

Fixes #3419.

Test plan

• git diff --check
• uv run python - <<'PY' ... PY validating a url_citation annotation parses as AnnotationURLCitation
• uv run python -m py_compile src/openai/types/responses/response_output_text_annotation_added_event.py