Assert the traversal redirect by property, not by status code

net/http's mux answers a literal ../ in the path with a redirect whose
exact code differs between Go 1.25 (301) and 1.26 (307), so pinning the
code failed CI while passing locally. Assert what the test is actually
about: the request is redirected rather than handled, and no kids-tree
entry is created.

internal/server/server_test.go

@@ -273,9 +273,6 @@ func TestMutationRejectsBadSectionAndName(t *testing.T) {
 		{"/api/library/movies/Ghost/mark", http.StatusNotFound},
 		{"/api/library/movies/..%2f..%2fetc/mark", http.StatusBadRequest},
 		{`/api/library/movies/..\..\etc/mark`, http.StatusBadRequest},
-		// A literal ../ never reaches a handler: net/http's mux normalises the
-		// path and redirects, so no mutation runs.
-		{"/api/library/movies/../mark", http.StatusTemporaryRedirect},
 	}
 	for _, c := range cases {
 		rec := e.do(t, http.MethodPost, c.path)
@@ -283,6 +280,22 @@ func TestMutationRejectsBadSectionAndName(t *testing.T) {
 			t.Errorf("POST %s = %d, want %d (%s)", c.path, rec.Code, c.want, rec.Body)
 		}
 	}
+
+	// A literal ../ never reaches a handler at all: net/http's mux normalises
+	// the path and redirects instead. The exact redirect code has varied across
+	// Go releases, so assert the property that matters — it is a redirect and
+	// no mutation ran.
+	rec := e.do(t, http.MethodPost, "/api/library/movies/../mark")
+	if rec.Code < 300 || rec.Code >= 400 {
+		t.Errorf("POST with a literal ../ = %d, want a redirect (%s)", rec.Code, rec.Body)
+	}
+	if _, err := os.Stat(filepath.Join(e.root, library.KidsTree, "movies")); err != nil {
+		t.Fatalf("kids movies dir disturbed: %v", err)
+	}
+	entries, err := os.ReadDir(filepath.Join(e.root, library.KidsTree, "movies"))
+	if err != nil || len(entries) != 0 {
+		t.Fatalf("traversal attempt created kids entries: %v %v", entries, err)
+	}
 }
 
 func TestArtProxyStreamsAndHidesTheKey(t *testing.T) {