feat(runtime): add streaming support to LlmDriver trait

- Add StreamChunk and StreamEvent types for Tauri event emission
- Add stream() method to LlmDriver trait with async-stream
- Implement Anthropic streaming with SSE parsing
- Implement OpenAI streaming with SSE parsing
- Add placeholder stream() for Gemini and Local drivers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -27,6 +27,7 @@ rust-version = "1.75"
 tokio = { version = "1", features = ["full"] }
 tokio-stream = "0.1"
 futures = "0.3"
+async-stream = "0.3"
 
 # Serialization
 serde = { version = "1", features = ["derive"] }