fix: resolve ruff lint errors in Phase 2 code

- Move intent imports to TYPE_CHECKING block in graph.py (TC001)
- Rename test classes to CapWords convention (N801)
- Fix line length violations across test files (E501)
- Auto-fix import sorting (I001)

backend/tests/unit/test_ws_handler.py

@@ -222,7 +222,9 @@ class TestHandleUserMessage:
         im = InterruptManager()
 
         sm.touch("t1")
-        await handle_user_message(ws, graph, sm, cb, "t1", "cancel order 1042", interrupt_manager=im)
+        await handle_user_message(
+            ws, graph, sm, cb, "t1", "cancel order 1042", interrupt_manager=im,
+        )
 
         # Interrupt should be registered
         assert im.has_pending("t1")