Add TurnEnd event type to wire protocol for explicit turn termination.
When wire protocol version >= 1.2, set UnexpectedEOF status if turn ends
without receiving TurnEnd event.

Changes:
- Add TurnEnd struct, EventTypeTurnEnd constant, and unmarshaler
- Add PromptResultStatusUnexpectedEOF status constant
- Add wireProtocolVersion field to Turn and Session structs
- Handle TurnEnd in traverse loop to properly end turns
- Update mock_kimi to send TurnEnd in all prompt handlers
- Add unit tests for TurnEnd handling and UnexpectedEOF detection
- Add integration test for explicit TurnEnd