Implement real Jupyter backend with jupyter-client

- Replaced mock exec() implementation with real Jupyter protocol
- Uses jupyter-client (host) to connect to ipykernel (container) via ZMQ
- 1:1 mapping: one container per session, one kernel per container
- Proper Jupyter message protocol for code execution
- Kernel lifecycle management (start, execute, shutdown, restart)
- Namespace inspection via introspection code
- Idle kernel cleanup
- Connection file management
- Backed up old implementation as kernel_old.py

TODO:
- Container image needs ipykernel installed
- Need to implement proper port mapping for ZMQ
- Need to mount connection file into container
- Add better kernel readiness check
- Implement restart_kernel properly with KernelManager
- Write tests

pyproject.toml

@@ -9,6 +9,7 @@ authors = [
 requires-python = ">=3.13"
 dependencies = [
     "fastmcp>=2.14.5",
+    "jupyter-client>=8.8.0",
     "podman>=5.7.0",
     "pydantic>=2.12.5",
     "pyyaml>=6.0.3",