fix(tests): update Dockerfile test to expect python:3.13-slim

docker/Dockerfile was bumped from 3.11 to 3.13 in 50076d7 without
updating the assertion, breaking the suite on main. Verified 3.13
compatibility by building the image and booting ComfyUI 0.27.0 in it
(custom node loads, DB migrations run, no errors) before updating the
pinned version the test enforces.

Fixes #20

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YArD9ZjBWKsAvazmS48amA

tests/test_packaging.py

@@ -210,8 +210,8 @@ class TestDockerCompose:
             "docker/Dockerfile not found — run T041-I to create it"
         )
 
-    def test_dockerfile_uses_python_311_base(self):
+    def test_dockerfile_uses_python_313_base(self):
         text = DOCKERFILE.read_text()
-        assert "FROM python:3.11" in text, (
-            "Dockerfile must use python:3.11 base image (CPU-only, no CUDA)"
+        assert "FROM python:3.13" in text, (
+            "Dockerfile must use python:3.13 base image (CPU-only, no CUDA)"
         )