fix: refuse a Redis URL on port zero

parts.port returns 0 rather than raising, since 0 is inside the range it checks,
so the URL reached .env and the worker and cache had nothing to connect to.

tests/deploy/test_native.py

@@ -231,7 +231,8 @@ class TestNativeUp:
         with pytest.raises(DeployError, match="too long to read"):
             _run(argv, _native_context())
 
-    @pytest.mark.parametrize("url", ["redis://localhost:notaport/0", "redis://localhost:65536/0"])
+    @pytest.mark.parametrize("url", ["redis://localhost:notaport/0", "redis://localhost:65536/0",
+                                     "redis://localhost:0/0"])
     def test_a_redis_url_with_an_unusable_port_is_refused(self, tmp_path, url):
         """urlsplit accepts it and rebuilds it verbatim; only .port notices, and the worker dies later."""
         argv = ["up", "--native", "--dir", str(tmp_path), "--yes",