step-3: add router refactor with streaming support (orchestrator.py)

- orchestrate_v3(user_id, message, context): classifies intent, returns
  (agent_name, agent_instance) — caller drives execution
- orchestrate_v3_stream(user_id, message, context): yields (agent_name, token)
  pairs; first yield is always (agent_name, "") as a domain-detection signal
- ChatAgent.handle_stream(): default implementation yields handle() result as
  one chunk; subclasses override for true token-level streaming
- Fix stale test_orchestrator.py assertions that expected a JSON final frame
  that orchestrate_stream never emitted

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

app/core/agent_registry.py

@@ -45,6 +45,16 @@ class ChatAgent(BaseAgent):
         """Process a user query and return a text response."""
         ...
 
+    async def handle_stream(
+        self, query: str, context: dict[str, Any]
+    ) -> AsyncGenerator[str, None]:
+        """Streaming variant of handle().
+
+        Default: calls handle() and yields the full response as one chunk.
+        Override in subclasses for true token-level streaming via _tool_loop_stream.
+        """
+        yield await self.handle(query, context)
+
     @abstractmethod
     def get_tools(self) -> list[Any]:
         """Return LangChain tool definitions available to this agent."""