Add a unit test for duplicate top-level compose volume names

The check itself already existed in load_compose_file() (same shape as the
already-tested duplicate network name); this was just missing coverage.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

tests/unit/test_compose_file.cpp

@@ -529,6 +529,21 @@ TEST_CASE("compose file: service volumes -- bind mounts (absolute-resolved, ro),
     CHECK_FALSE(load_compose_file(relative_target).has_value());
 }
 
+TEST_CASE("compose file: duplicate top-level volume name is an error", "[unit]") {
+    ScratchXdgDirs scratch;
+    // A duplicate YAML mapping key under volumes: -- same shape as the
+    // already-tested duplicate service name (libyaml keeps both pairs
+    // rather than deduplicating).
+    auto path = write_compose(scratch.path(),
+                               "services:\n"
+                               "  web:\n"
+                               "    image: busybox:latest\n"
+                               "volumes:\n"
+                               "  applog:\n"
+                               "  applog:\n");
+    CHECK_FALSE(load_compose_file(path).has_value());
+}
+
 TEST_CASE("compose file: validate_compose_external_state -- env_file existence, external network existence",
           "[unit]") {
     ScratchXdgDirs scratch;