Give Responses Lite prefix items stable IDs (#40962)

## Why

Responses Lite rebuilds its tools and base-instruction prefix for each request. Stable item identities let follow-up WebSocket requests recognize that unchanged prefix and send only incremental input.

## What changed

- Derive deterministic IDs for additional-tools and base-instruction items from the thread ID and each item's serialized payload.
- Preserve IDs when rebuilding an unchanged request or resuming a session, while changing them when the thread or payload changes.

## Testing

- Cover ID stability and payload/thread sensitivity in request construction.
- Verify resumed Responses Lite sessions reuse prefix IDs and WebSocket follow-ups use `previous_response_id` with only new input items.

GitOrigin-RevId: 3c8548b96979d7578865b0ccc51c147926823478