Emit standard Realtime output-item and content-part lifecycle events

[andimarafioti]

Problem

The Realtime endpoint emits response deltas, per-modality done events, and response.done, but omits the standard per-item lifecycle events:

• response.output_item.added
• response.content_part.added
• response.content_part.done
• response.output_item.done

The official Realtime text lifecycle places these events around response.output_text.delta / response.output_text.done and before response.done. The response.output_item.done reference also specifies that it is emitted when a response is interrupted, incomplete, or cancelled.

This omission predates #453. Ordered multi-item responses make it more visible because a client waiting for item completion only discovers the final items inside response.done.

Expected behavior

Emit the standard lifecycle for every assistant message or function-call output item while preserving the stable item IDs and response-wide output_index values already used by the endpoint.

Acceptance criteria

• Emit response.output_item.added when each response output item begins.
• Emit response.content_part.added before streaming a message content part and response.content_part.done after its modality-specific done event.
• Emit response.output_item.done after every item completes, including items exposed before interruption, incomplete completion, or cancellation.
• Emit response.done only after all applicable item/content lifecycle events.
• Keep IDs and output indexes consistent across lifecycle events and response.done.output.
• Add WebSocket and WebRTC protocol-sequence contract tests covering text, text → tool → text, and cancellation.