- Introduce `StreamingMessageState` to track full text, last tool calls, and completion.
- Add `StreamDiff`, `TextDelta`, and `TextDeltaKind` for describing incremental changes.
- SessionController now maintains a `stream_states` map keyed by response IDs.
- `apply_stream_chunk` uses the new state to emit append/replace text deltas and tool‑call updates, handling final chunks and cleanup.
- `Conversation` gains `set_stream_content` to replace streaming content and manage metadata.
- Ensure stream state is cleared on cancel, conversation reset, and controller clear.