fix: make multipart discovery and cancellation explicit and bounded

Signed-off-by: Feng Ruohang <rh@vonng.com>

internal/config/api/api_test.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2026 Ruohang Feng
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package api
+
+import (
+	"testing"
+
+	"github.com/minio/minio/internal/config"
+)
+
+func TestMultipartListingMigrationMode(t *testing.T) {
+	for _, tc := range []struct {
+		name, stored, override, want string
+		invalid                      bool
+	}{
+		{name: "default", want: "strict"},
+		{name: "explicit-migration", stored: "legacy", want: "legacy"},
+		{name: "environment-override", stored: "strict", override: "legacy", want: "legacy"},
+		{name: "invalid-stored", stored: "automatic", invalid: true},
+		{name: "invalid-environment", stored: "strict", override: "automatic", invalid: true},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv(EnvAPIMultipartListing, tc.override)
+			var kvs config.KVS
+			if tc.stored != "" {
+				kvs = config.KVS{{Key: apiMultipartListing, Value: tc.stored}}
+			}
+			cfg, err := LookupConfig(kvs)
+			if tc.invalid {
+				if err == nil {
+					t.Fatal("invalid migration mode silently accepted")
+				}
+				return
+			}
+			if err != nil || cfg.MultipartListing != tc.want {
+				t.Fatalf("mode=%q err=%v, want %q", cfg.MultipartListing, err, tc.want)
+			}
+		})
+	}
+}